I bought a PF-500 early in their release history. It worked fine for the first 6 years, then went into fault after an hour or so of playing, and had no output. After it cooled down, it would work for another hour, and so on. The fan was operating fine, and it did not feel particularly hot, so it was likely a faulty temperature sensor. It was going to cost almost as much to repair it as it was to replace it. I really liked the amp, so I bought a new one with a new rev main board, after reading the newer revs are better. It's been working fine for a few years.
About a year ago, I checked into the price of a replacement main board, and found one for $189. I replaced the main board in my original unit with a current rev board, and now I have two that work great! You can buy new replacement main boards on Instrumentalparts.com. It's a very easy replacement.