I bought my first A4000 from a very dodgy fellow, whom I didn't trust at all. I ran it and checked it all out before I gave him any money, all with no problems.
As soon as I got it home, it started to play up, running slower and slower as it warmed up, until about an hour after it was switched on, it would just stop. I'll bet he was glad to be rid of it and laughing at my stupidity.
It didn't take long to find the faulty chip with some freeze spray. I took it to a local C= repair agent, who replaced the chip on the 3640 (or was it the mobo?). Total cost; AUS$12.00 :-D
Although it's now in a tower, I'm still using it, 10 years later, never missed a beat since :-D
tony