It may not be a 'hardware' problem in the radio at all.
There was a phase when one of my MRKs decided it wasn't going to write successfully with anything less than a fully & freshly charged battery attached. That was with a Price "RIB-less" cable which I think takes its power from the host radio.
If you're using a TQ3370 interface, then check that it's getting correct voltage... spec says 7.5 to 16 volts DC. Use a regulated supply... the input filtering of the TQ3370 isn't that great.
If you're just writing a personality to the radio, then the 12 volt Vpp shouldn't be needed. When writing firmware to the radio as well, I think early versions of the MRK firmware required Vpp to be set, and some RIB-less cables don't have a Vpp line.
But by far the most common problem with programming of radios is the PC & OS itself. With the brand of radio I usually mess around with, they're hyper-critical of running the programming software under the correct OS, and even programs or processes running in the background can mess up a device call to the COM port to the point where programming software just won't work. Some RIB-less cables source their power from the COM port itself, and its been found some COM ports just won't drive these reliably; likewise it would be a bad idea to do anything to a GE et al branded radio with a COM powered RIB-less cable (if one exists?)
Try programming from a different PC first. That might solve all your problems.