Based on everything said so far (especially since you got errors without expansion cards), I'd imagine, like others have said, the memory is intermittently dying. However, the memory should be easily replaceable, provided the error isn't with the first memory bank (at which point you'd have to de/resolder the memory from the board- nothing more relaxing than an hour of sold). The IBM PC takes 4164 memory ICs (please correct if I'm wrong) as I recall, and can be found on Ebay at modest prices in bulk if you look for it:
http://www.ebay.com/sch/i.html?_nkw=4164+ram&_sacat=0&_odkw=4164&_osacat=0
BEFORE THAT however, my suggestion for now is write down any error codes that pop up, and consult the PC technical reference manual for clues on the problem- as I remember, there are sections for diagnosing hardware errors using th return codes (the numbers BESIDES 201- memory error, 301- keyboard error, etc).
It could also possibly be a power supply error- idk if the PC returns error codes for power supply failure, however (does anyone else know?).
http://www.ebay.com/sch/i.html?_nkw=4164+ram&_sacat=0&_odkw=4164&_osacat=0
BEFORE THAT however, my suggestion for now is write down any error codes that pop up, and consult the PC technical reference manual for clues on the problem- as I remember, there are sections for diagnosing hardware errors using th return codes (the numbers BESIDES 201- memory error, 301- keyboard error, etc).
It could also possibly be a power supply error- idk if the PC returns error codes for power supply failure, however (does anyone else know?).