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I have built several VHF amps (2 meters) in the 20 to 50 watt class and also found that the coax can really screw up measurements. One I remember would give almost no gain. Turns out that I had grabbed a piece of CB grade coax to couple the exciter to the amplifier. A three foot length of this stuff gave about 10 dB loss. Got some good Belden RG58 and the amp worked like it was supposed to. VHF is a different world.
One subtle problem that comes up with VHF power amps is the power supply and/or the output amplifier can oscillate. This could be affecting your audio and if it is happening you will definitely be interfering with many stations.
My sad experience with this was with a mobile amp powered from the car battery. When keyed, there was a 5MHz. oscillation at the amplifier power terminal. I fixed it as described below.
Standard practice is to bypass the supply right at the final output transistor power source (the collector choke) with a series ferrite bead and three caps in parallel to the gound plane (10 uF, 1 uF, and .1 uF). I also resorted to a neutralization feedback network from the collector to the base. This was a small cap, inductor, and resistor in series. Don’t remember the values but the ARRL book has examples.
I found the problem when I checked the 13.8 volt VCC with a scope. The oscillation was obvious.
Hope this helps.