I don't think it's a good idea to unplug and plug fans in while the system is powered up. Too many bad things could happen.
It's simple enough to unplug the case fan before you power up. See if that makes a difference. But, do not unplug the CPU fan!
Have you checked the mounting screws for the PSU to make sure they are tight? Are all your drives secured tightly?
I agree wholeheartedly that the HSF should be replaced. I assume you're using the HSF that came with the CPU?
An extremely good and inexpensive heatsink right now is the Thermalright SK-7. Use an 80mm case fan of your choice with it. I would recommend a fan with minimal cfms of 40 or more. Bestbyte has some good 41cfm Top Motor fans for dirt cheap. Or, you could go with a 47cfm Panaflo UA1 or the 53cfm Mechatronics.
Your CPU temps shouldn't be that high for a stock AXP 1700+. When you replace the heatsink, get yourself some Goof Off found at Walmart for cleaning off the old TIM from the core. Then follow that up with some 91% isopropyl alcohol for a final cleaning. Use a coffee filter or other lint free cloth to give it that final wipe before applying ASIII.
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