The laws of thermodynamics apply equally to GPU and CPU. Please note that just because you aren't doing anything, your system may not be idling. proceesd can and will activate in the background and your CPU will not downclock immediately when you arrive in Windows ... typically, the CPu will remain at full speed for 2 - 5 minutes before dropping down to 800 MHz or so.
It most certainly is possible for temps to vary that fast ... run a stress test and then stop, you will see 20C or more drop in < 1 second. Yes, the temperature of the coolant will vary slowly ... but the surface temp of the CPU/GPU will drop extremely fast as soon as heat stops being applied because the thermal mass of the CPU / GPU surface is miniscule compared to that of the block that's cooling it.
As for monitoring tools, for CPU, MoBo and everything else, HWiNFO is the best utility for temp / voltage monitoring I have ever used ... and since installing I haven't used anything else. Even provides GFX VRM info. I still use AB for GFX, ... I dunno, force of habit maybe.