98 Mercury Sable AC recharge problems

[NextBestThing]

Hey everyone!

OK! My girlfriends Mecury Sable is blowing out cold air with the AC on. The blend door is fine and all components of the system seem to be in working order.

It's a POS used car, already had the transmission replaced and the intake manifold was screwed over.

I hooked up the R134a to the low side and started filling it up, the compressor kicked in, thats a plus. The air went from hot to warm after a few minutes of charging.

The problem comes here: The gauge goes wacky, it bounces up from 15 to 30 and back down and back up. What would cause this? Would it just be a leak I need to find and fix? Could it just be the gauge? I hooked up the gauge to my 02 Neon and it said I was overcharged a bit. The dealership takes care of that car so that makes me a little uneasy about both the gauge and the dealership...

What do you guys think?