Rick,

Yes, it could be bad coax or bad connections along the line from the 
transceiver to the antenna.

If you are transmitting during the times between readings, it could, also, 
be heating in the MFJ tuner.

I have gone through several MFJ tuners which overheated and softened the 
insulating strips in the tuner coils, thus changing the induction of the 
coils.  With MFJ you probably should have a tuner rated for twice or three 
times the power you are putting out from the amplifier.

Good luck.


73,
Gale
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