[TowerTalk] Tribander problem

[RLVZ at aol.com]

Here's the problem: A Hy-Gain TH3-Mk4 tribander works fine at 100 watts... 
but with a KW the SWR goes up when used for an hour or so at 50% duty cycle.  I 
pulled the traps and all looked well except for a couple loose screws which I 
tightened but it didn't solve the problem.   I have also tried replacing the 
balun without success.

Any idears what could be the culprit?

Thanks!

Dick- K9OM