Intermittant A/C operation.

[Rhino]

I've been dealing with my DD's poor performing A/C for awhile, and I've finally decided to repair it. The system performs marginally ok, but I'm sure it could be better. I'm looking for suggestions into what I should start looking at.



It's an '03 Avalanche with climate control. The A/C seems to work ok, however it has periods of time (A few seconds out of every minute) that will cycle warm.
Hooking a set of gauges up to it, I've noticed steady pressures that were within spec found in eSI. Unfortunately I can't remember exact values. What I find odd is when the air exiting the vent warms up, the needle on the high side gauge will start climbing. It will get to roughly 500 psi before slowly decreasing and the system will start performing again. The low side decreases a little, however not nearly as dramatic.
A neighbor used to do A/C work at a dealer decades ago and said it was possibly water in the refrigerant. I did have to add a little to it right after purchasing the truck. It worked great for a year before it starting exhibiting this behavior. I do believe it has the proper charge, however I'm not certain of it's oil level.

I plan on replacing the drier and orifice tube prior to vac'ing and recharging. Is there anything else I should do while the system is open? Is there anything I can do at home to remove the oil and add the proper amount? Due to the extreme high side pressure, I don't feel it is on the control side of things, however please feel free to throw out any suggestions if you have any.