Strange problem with HP 9100A

Hello, I just acquired an HP 9100A in great condition with no signs of abuse but it has a strange problem: although the numeric input keys and four functions work fine, the transcendentals and other keys on the leftmost pad do unexpected things. When in the radian mode, many of the keys (like sin x, cos x, tan x, e^x) perform an x<>y, but even more strangely, in the degree mode pressing these same keys blanks the display and hangs the machine. After such a hang, when I cycle power the display lights up with 9.999999999 99 in the x register. Also, storage registers a, c, e, and f seem to work fine, but registers b and d return bizarre values. I've examined the machine thoroughly and find nothing obviously wrong or missing. I've reseated all of the cards, but to no avail. When I enter the program mode, pressing each key seems (on cursory inspection) to produce the correct key code. Any ideas? TIA, Stan