Do these messages occur with the card in the slot ? If so, remove it.

The battery in the card only lasts a few years, so it sounds like it needs changing.

For the bad checksum problem, you could try a "clear all" (8 key held down at startup).

But, since the battery that power the internal ram only last about 8-10 years, this may need replacing as well. These are hard to find.

Of course, given the number of errors, you may still have some hardware problem.  It wouldn't hurt if you had a local tech look at it.