The other stuff, however, sounds like a troubling continuation of
previous problems. Maybe it is chipset or CPU or board related. On the
other hand, changing the memory modules seems to have changed something.
Scary stuff, that. I don't think it likely that the hard disk or its
interface is at fault: your symptoms sounded so transitory that they are
likely to occur in-memory.
Stuff like a bad extension card or a damaged PCB track or terminator on
bus line D2 or a solder splash adding unwanted capacity can also lead to
such unreliable effects through reflections/delays. Today's computers
are annoyingly high frequency devices.
At any rate, the symptoms are quite clearly pointing to a hardware
problem