Just a quick thought, when ever I'm in doubt with an old(er) PC acting funky, the first thing I'll do is replace the CPU keep-alive battery on the mobo.
Its a cheap and easy thing to try (and often effective) plus it forces you to closely examine the mobo and cards seated on it. The battery isn't as much of an issue on newer systems, however an old, dead battery can still cause problems related to DC power.
