|"The CPU overheated in the past due to technical problems with the fan"|
The Athlon XP has no thermal protection, if it gets too hot, it will fry. As stated above, the thermal paste & heatsink MUST be installed correctly. Too much paste is as bad as too little or no paste. All you need is a tiny dab about the size of a grain of rice & it should have been spread thinly & evenly over the core.
I'm a little suspicious about your claim that your overclocked an 1800+ (1.53GHz) to 2400+ speed (2.0GHz). That's quite a jump. Assuming the multiplier is locked at 11.5x, it would mean you cranked up the CPU frequency to approx 175MHz? Make sure you maintain a 1:1 ratio. That is critical to good performance with a socket A system. If you're running the RAM frequency faster than the CPU frequency, you're actually hurting performance. RAM timings are important too. I'm sure you had to increase the CPU voltage to be able to run the CPU at that speed, so that's probably where the high temp is coming from. Or it could be your old board reported the temps on the low side or the new board may report temps on the high side. Where are you getting the readings from? And why did you change the board to begin with? The AXP is very outdated & not worth throwing money at.