can you check the coax jumper from the ts-50 to the at-50?
perhaps it has a bad connection at the pl259's.
If you attach dummy load to the at-50 will the problem repeat?
Have you replaced the lithium battery in the ts-50 recently?
Sometimes when these batteries reach end of life the radio
can develop problems that are solved with a new battery.
Here is a great group for TS-50/60 fanshttp://groups.yahoo.com/group/Kenwood_TS50/