Look at "Fixing and Initializing Internal Memory" on UM p.53 to clear the bad checksum message.
If it comes back later, change the battery again. If this does not fix it, or it comes back immediately, you have a hardware fault.
If you actually do have a hardware fault, it has to go for service – there is no magic fix. It can be a bad memory chip, or faulty parts in the shutdown circuitry.
