i second the RAM suggestion - HL2 skipped constantly when I was using 512MB of RAM. Not sure if it's any different between A64 and P4, but that's a likely culprit - fortunately, 1GB kits are like $90 on newegg right now, so it's pretty cheap fix.
I doubt it's overheating cause as far as i know, the P4 throttles well before overheating would cause a freeze. That means a percentage of its cycles are spent idle, cooling off, so you'd notice everything slow down, but not freeze. For example, with 50% throttling (the highest i think it goes) a 3GHZ P4 would be an effective 1.5GHz P4, so your games would crawl, but not stop.