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the reset side fails to work correctly and I get a lot of black screens where pin 14 of the 8255 stays at 0v until the picture comes back.
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No, this is a problem.
Pin 14 (V-Gate out from the 8255) is not correctly set up in the RAM test code to output a logic '1' to make V-Gate high - as you are in contact with the author of the RAM test EPROM maybe you could alert him to that problem or just draw his attention to this thread - he obviously has very good knowledge of these machines so we'd welcome his direct input. However, the resistor you added should compensate for that issue in the meantime.
It is possible that in the circumstances you describe the 8255 is being part initialised with that port pin set to be an output and then driven low, which will have the effect of driving the V-Gate line low and turning the video off, hence blank screen.
There are only officially two things connected to that line, 8255 pin 14 and IC25 pin 1 (plus the added pull up resistor). First check the resistor you added, make sure the soldering is good and that you did connect the other end of it to a known good +5V point.
To eliminate the 8255 as the source of the problem remove it, carefully bend pin 14 out to one side so it no longer goes into the socket and then re-insert the 8255 still with that pin hanging out to one side. Does that allow the screen to stay on consistently?