RV1 is to set the quiescent (no modulation) PA current. RV2 sets the modulation depth.
I meant to change RV2 to 100 Ohms value and connect R14 across it's track (video input to ground) because the standard for analog video cables is a 75 Ohm termination. The 100 Ohm potentiometer and 82 Ohms will give about 45 Ohms. If you rewire it and change R14 to 270 Ohms it will present 73 Ohms load which is better.
You can adjust both controls by watching the picture, feed it a steady video signal, ideally color bars and gradually increase R2. When it starts to tear/roll or the difference between lightest or darkest bars starts to reduce, adjust RV1 to try to correct it. when you have found the best position for RV1, reduce RV2 to drop the contrast to a more reasonable level. What you are trying to achieve is the PA current that allows maximum amplitude before the top or bottom of the waveform become distorted.
Where are you monitoring those video waveforms? The best way to adjust it is to use oscilloscope inputs, one on the video source (at the input of the circuit) and one on a diode probe across the RF load (antenna socket, across a dummy load or antenna). If you set the scope to XY mode it should draw a diagonal line or at least a steady diagonal shape. RV2 should set the length of the line and RV1 should set how straight it is. If RV1 is set wrongly, the top or bottom of the line will go flat, you adjust the controls to get the longest straight line possible.
If the PA seems to go dead, the most likely reason is lack of current, do you have a heat sink on Q6?
Brian.