Hello all, my pc is dell optiplex 980 MT. I know my pc is quite old and i've been using it ever since 2010. The problem is my pc gets extremely slow and even freezes after a while. e.g after browsing or even being idle etc. It's been this way for about 1 year. I don't use my desktop as often as i used to like the time when i didn't have a laptop. So heavy load is not an option. Anyway, what happened recently is my original dell 305 watt psu exploded, and fortunately it didn't take other components with it. I bought a cheap dell psu, which is 265 watt from an unreliable service man and i suspected that the cheap psu could be a cheaply refurbished one, but for the sake of availability and budget i got that one because i live in a developing country, so purchasing from abroad takes some time. The 265 watt supply worked, but pc lag and slowness didn't get better, and after about 10 days later pc shut down with "critical thermal event" error. I tried to turn on my pc and it did turned on, but freezed completely in a starting up phase or after startup process. I thought dust caused that so i cleaned the heatsink and reapplied new thermal paste. Dust was indeed bountiful and old thermal paste was dry. So it seemed that i solved the problem. Then when i turned my pc on, it started and the cpu fan was speeding fast, but after a while it became normal, and pc seemed not lagging or slowing, but after a while about 20 min use, a strange buzzing/rattling noise heard on my earphone and even from the pc. I quickly shut my computer forcefully. then i tried to start my pc 234 error massage was displayed on front panel and amber light was on. I thought my pc was done, but after a while when i clicked the power button my pc turned on normally. In my opinion main culprit is that cheap psu, but could be other things. Could downgrading from 300watt to 265 watt cause this kind of problem? My gpu is ati radeon hd 5450 1 gb. What do you think causing my computer act strangely? Is it wise to buy a good and new powerful psu with 450 or 500 watt from Amazon, betting on blindly that psu is the main cause. Ps: i'm a complete noob for computer hardware knowledge, so please help me.