Weird behavior HP-33S C key

[Kees van der Sanden]

A recently acquired HP-33S shows weird behavior.

After the calculator is turned on the C key stops responding
after about 30 seconds. All other keys still respond.
Also turning off the calculator by shift-C is no longer possible.

When the calculator is turned off by the shift-C combination
(within 30 seconds), it can be turned on again by the C key within 30 seconds.

Only a hard reset brings the C key temporarily back to live.

The calculator has fresh batteries.