Nikon D50 DSLR freezes [locks up] after it warms up - most likely explanation?

This camera is somewhat of a classic, and has given many years of reliable service, but recently has developed an issue.

when it is turned on from a cool state, it works perfectly for a few minutes (less if used intensely, more if used sparingly).

after it has been on a few minutes and has begun to warm up (from the heat dissipated by the electronics), it will:

- freeze as menu selections are being made, or
- freeze right right after the shutter fires (no picture is recorded), or
- freeze in the middle of a "playback slide show" running on the lcd, or

etc.

[the camera has never ran abnormally hot, just slightly warm]

when it freezes, the top-side lcd panel remains lit, and no error message is displayed. If the lcd screen is on when the freeze occurs, it also remains lit. If the lcd screen is off when the freeze occurs, it stays off.

once camera has frozen, there's no response to pressing any buttons, except the shutter button, which can activate focusing motion, and sometimes even fire the shutter once (and only once), with no picture recorded.

[the contents of the memory card are unaffected by freeze-ups, and the photos stored there are always preserved for later viewing or downloading.]

if i turn the camera off and then back on, it starts working normally again, but will freeze up again in a pretty short time (usually less than a minute or two).

as it gets steadily warmer, there comes a point where turning it off and on no longer has the effect of restoring normal camera function.

but then i can still get it working normally again by removing and reinstalling the battery. However then it will quickly freeze up again, usually in an even shorter time than before (maybe thirty seconds or less).

after a few times of removing and reinstalling the battery, the camera begins to have difficulty recognizing the memory card, at which point the only thing i can do is let the camera cool down to room temperature.

then it will work normally again, until it starts to warm up.

what are some of the most likely explanations for this behavior?

For all strange behavior, I start with the battery. If you've had it for a while, replace it. Rechargeable batteries do wear out. After that you need to contact Nikon and be prepared to send it in for repair.