Not a problem at all, happy to help.
Is the actual cooling block (the entire unit) loose, or is it just the fan? Which CPU cooler is it?
If it's the entire unit, then there's every possibility that the cooler is not secured correctly onto the bracket plate, and as such is causing the CPU to overheat and thus reduce it's performance due to thermal throttling.
It's also possible that if this is the case, and there is a slight air gap between the bottom of the cooler and the top of the CPU (where the paste sits) then air can get into the paste and form a slight "air bubble".
The best thing to do in my opinion, would be to firstly buy some more thermal compound/paste, go for a decent brand if you can, something along the lines of Artic MX-4 brand, or some other decent brand if you can get it. They're not too expensive, some years ago I bought a pack of 4 tubes of it for about £15.
Firstly, turn off the computer and switch off the power supply at the back, then disconnect the IEC/power lead.
Ground yourself on anything metal, in my case, I use my bed which has a metal frame as it's right next to my desk where my PC is.
Open the side of the case, remove the CPU cooler using a screwdriver, most coolers will use a typical "cross head" Philips screw design, some use flat (horizontal) designs and some have their own proprietary designs.
Remove the screws, or in some cases, thumbscrews from the cooler unit and put them aside.
Next remove the cooler unit itself, and put it to the side, preferably on some old newspaper or tissue so you don't get thermal compound all over the floor/carpet, in case you haven't seen it before, thermal compound is basically a grey sticky paste, grey is the most common colour i've seen, but i'm sure you can get other colours out there, so yours may be different.
Clean the bottom of the cooler, the plate that actually sits on the CPU, using a wet wipe or slightly wet cloth, then once all the compound has been wiped off, dry it.
Next, ground yourself again, as you are about to touch the CPU - just briefly touch anything metal nearby, preferably something metal that is actually on the ground.
If you can, without removing the CPU, wipe it clean with a cloth, clear off all of the thermal compound, and then make sure the CPU is dry. I say to use a wet cloth as that removes the existing thermal compound more easily than a dry cloth, make sure the cloth is slightly wet and not soaking wet, and also don't touch the underside of the CPU where the contacts or pins are with the wet cloth. If you need to remove the CPU to do this, that's fine, but just be careful handling it, and if you're sat on a carpet don't shuffle around on the floor as this increases your chance of building up static in the body which could kill the CPU.
Now, take your new thermal compound, and squeeze a small amount onto the CPU, a small amount about the size of a garden pea, just a small ball in the centre, there's no need to smear it all over the CPU, the cooler will do that for you when you re-seat it.
If you needed to remove the CPU to clean it, then reseat it, and close the bracket, if you didn't need to remove it, even better.
Next, take your CPU cooler, and place it back ontop of the CPU, try to align the cooler bracket (where the holes are) with the holes in the plate on the board itself before you press it down onto the CPU, so that you don't have to move the cooler around too much when seating it, any excessive movement could cause small air bubbles to get into the paste which reduces it's effectiveness.
Next, reinstall the CPU cooler screws, do opposite corners first, for instance, do the top left one, and then the bottom right corner one next, or bottom left and top right one first, this spreads the paste across the chip and also helps secure it in place more evenly.
Do both of these screws up, but just "finger tight" so to hold it in place, next put in the remaining two screws into the other two holes, and finger tighten them.
Then tighten them fully one by one, in opposite corners, until it's secure and the CPU cooler doesn't move around, it should sit solid and not slide about or be able to move slightly from side to side if it's securely fastened down.
Now, depending on what cooler you have, if you had to disconnect any wires from it, to remove it, re-attach those, most stock coolers or cheaper coolers won't have anything connected other than the standard CPU COOLER header which usually goes to the fan header on the motherboard which is usually labelled "CPU_COOLER" or "CPU_COOLER_1", connect the cable from the cooler back to this header on the board, if you have a liquid or water cooled system, then you may have a wire going to the CPU cooler block for software monitoring and control purposes as well as a cpu cooler header.
Now you can plug your PC back in, turn the PSU switch on, and power it up, I would personally leave the side case off at this point just to make sure it works, otherwise you'll have to take the side cover off again.
If it works as expected, and doesn't keep slowing down, then re-attach the side panel.
Job done!
This video should help a lot - it's easier to see on a video than it is to explain it.
View: https://www.youtube.com/watch?v=Sog0M9OrlME
Hope this helps.
- Will.