[Elecraft] [K3-KPA500] SWR jumps up with 5 second key down

[ai6ii]

I have installed a new antenna and have strange changing swr. When testing
with RigExperts meter I find good resonance at say 15m (21.200). With the K3
at 100 watts reading is 1 bar on K3 meter and 1.0 on KPA500 (STBY). When I
bring the amp online and drive it at 200 watts, all is still good even with
a 15 second key down period. Driving the KPA500 to 275 watts, still okay
with key down, although the KPA500 meter now shows 1.1. BUT when I increase
the drive a little more (300 watts) the swr jumps up to 2.5+ and the KPA500
shows REFL HI and drops into STBY.  

And here is the kicker...NOT ALL THE TIME! 

So I think it has to be in the coax. But the 75feet of RG213 is brand new
premade from Cable Xperts with the connectors.

I will try a different coax. Any other suggestions?

..mike

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