I have a King KT76 transponder that has picked up a little problem. It
takes about 10 minutes before it starts transmitting the mode C
information. ATC is getting the transmission alright, there is just no
altitude information.

It started doing this about a week ago and has been ever since. Once it
starts transmitting mode C, it seems to work flawlessly.

Any idea what is the problem?

Has anyone see this before?

My biggest concern is that it may stop altogether. I do a lot of
instrument flying, so that would not be good.