Had a new duron 800 that did the same thing, tried a bunch of stuff too.  If the whole computer freezes (which is what i think you mean by "lockup", and not just the program, it might be the heatsink not being big enough.  I thought I had a pretty big one, but I took it back to the guy I bought it from, and he slapped on some thermal grease, bigger heatsink, and HUGE fan after talking with some other people.  Now it works fine.  Just my experience. 
edit:  p.s. did the same thing about using my old HD and old generic memory.