Just to clarify, does actual the pressure in the keg go down, or does the guage register a much lower pressure after you pour a glass?
If the pressure goes down drastically, you should hear a large amount of gas going into the keg. This would be caused by a leak somewhere in the system. If there is only a slight noise of gas flowing into the keg, I suspect you have a bad pressure guage. You can pick up a replacement at Grainger for about $10.
Just my $0.02,