wayoverthere

Didn't want to clutter up your MXT500 thread with any more off-topic, but it's pretty definitely the radio. 

External meter agrees on voltage output from the PSU, and I took the Btech and the same antenna out to the garage ,stuck the antenna on top of my toolbox, and wired it to an extra car battery I have around (that stays float charged...it was pulled chasing a starting issue that ended up not being the battery), and it's doing the same thing.

I also did some heavier testing on the radio last night in the wee hours...no fluctuation on low or mid power.  Also ran it through some longer transmit cycles a minute at a time with minimal breaks, enough to get the fan running.  It seems to lose power as heat builds, it'll bounce back kind of slowly if I unkey and key again, and  it eventually steadied out around 25 watts. 

Transmit time is a lot less than receiving, and I rarely use high power as it is, so I'll probably just run it till it dies and either pitch it or use it as a scanner after that.  For on the air I'll just add an antenna switch so my 70cm vertex has access to both antennas.  Seems like a better option than digging out the MXT115