I'll bet the motherboard problem is that some of the capacitors in the power regulation section are failing. Look for any that are bulging and/or oozing lovely looking brown goop? If that is the problem, they can be replaced:
silentpcreview.com | View topic - A mini re-capping guide
On the heatsink -- how large is your case? Is it a midtower? I think the Scythe Mugen 2 is an excellent heatsink -- the fan is very quiet, and the mounting bracket is a bolt through design. And it is a pretty reasonable price:
Newegg.com - Scythe MUGEN-2 SCMG-2000 120mm Sleeve CPU Cooler - CPU Fans & Heatsinks
It is a big 'un!
