I would suspect that KT3 has a bad contact in that it is switching, but one of the two are not connecting causing an open line to the antenna. If this is true, I would be sure to fix it before doing any long transmissions. With the PA power connected, verify that you are drawing current on the PA power input. You should see current draw on transmit. Then I would verify that the contacts switch on KT3 with the PA power disconnected. This can be checked with just your dvm.
NOTE: with the PA power disconnected, you can key the transmitter with either the mic or the CW key to verify the relay switching.
How are you verifying the power out, and more importantly, which version? KT3 is for the v3 or v4 ubitx on 80 meters. KC1 is the relay on the v5.