intermittent AC, is it an electrical problem

I have a 2004 chevy venture. the A/c has been intermitently working for a while. at first I
thought the refrigerent was low, I added some but it didn't help. I took it to a mechanic
friend. he told me that I had it over charged. He bleed some off and it started working. A
few days later we went on a trip and the alternator died. after that was fixed the A/c
wouldn't work again. we were out of town so I bought a A/c charge kit that had a hose
with the low side gauge. the pressure said about 100psi. I thought that was high(not
realizing that it was because the compressor was off) so i bled some off but stopped
when it didn't help. when we got back in town my mechanic found it was from the
computer getting hung up when the alternator died. we unhooked the battery positive and
grounded it for a minute and the AC started working again. He said the pressure was a
little low and that I should add some. I did, but then the A/C started working intermittently
again. I may have added too much so I bled some back off and checked it was the low
and high side gauges but it didn't help. basically I think the pressures good, and I don't
think it has a major leak. it has been working less and less, now it isn't working at all.
with the AC on i just checked for 12v to the compressor and it was 0v. what electrical
items should I most likly look at?