Chevrolet - Lumina :: 1999 - A/C Manifold Gauge Readings Fluctuating

The A/C on my hand-me-down '99 Lumina has never been outstanding, but has kept the Florida summers at bay. I recently turned it on again (I only ran it once or twice this winter) and the system was empty. I refilled it and it was empty again in 72 hours. I was hoping the seals had just dried out, but it continues to empty after several other attempts to recharge it, including adding 134a with lubricants. I added 134a with UV dye in it and the only leak I found is around the compressor clutch. I accept that the compressor probably has to go, but am puzzled by the manifold gauge readings taken when I added the last can. I just don't want to put in a new compressor and find out that wasn't the only problem.

With the system off, both High and Low sides read just under 70. (Normal Low and High side operating pressures for this car are listed at 42 and 340 respectively.) When the system is turned on, Low drops to 30 and High goes to 80 and there is no air cooling. After 30 seconds the High side slowly climbs to 200 and the Low side slowly drops to 26, with almost no air cooling. When the High side hits 200, it drops back down to 80 and the Low side goes back to 30. After 30-45 seconds it does it again and keeps repeating. The compressor clutch stays engaged the whole time. I looked in the Haynes A/C Techbook as well as online and can't find anything that resembles this description. If this is just symptomatic of a bad compressor, or is there more evil afoot? Also, while I have the system open are there any parts I should replace at the same time given the age of the system?