Intermittent working A/C on 2002 Chevy Venture

I have a 2002 Chevy Venture with rear A/C. In typical 98-02 Venture fassion, the condenser developed a leak in the lower passenger corner. I had the condenser replaced. At the time the condenser was replaced, the dryer was unable to be removed without damaging it as well. Therefore, the dryer was also replaced. The A/C never really worked great after that, but it was manageable on warm, but not too hot days. This spring, I decided to get serious about it and look for the problem. Hooked up a good set of gauges and noticed 105 on the low and 110 on the high. Everyone I talked to, and everything I read online, stated that if those pressures converge, it is more than likely a bad compressor. Replaced the compressor, added 8oz of PAG 150 per the compressor sticker. Vacuumed the system at -30 psi for 50 minutes, then put in 3 12oz cans or R134a refrigerant. The system calls for exactly 36oz with rear A/C. Took into considerations on a recommendation from a mechanic friend that the guage hoses probably held about 2 oz of that refrig. and added a bit from another can as well. Well, here is what I have now. On an average day such as today, you get in the van in the early morning when it is cool and relatively low humidity and the A/C blows so cold, the kids ask for me to turn it down and/or off. However, as the day goes on and the humidity and temperature rise, the A/C performance decreases until in the hottest parts of the day, approx. 1PM - 5PM, it barely cools at all. It is not warm, but it sure is not cold. It is mildly cool on high fan speed. Took it over to a buddys mechanic shop today and had the pressures read with his set of guages. 43 on the low and 220 on the high. The van had just been driven quite a bit and the outside ambient temp was around 85 with approx. 55% humidity. This evening, I can almost guarantee you that the kids will be asking to turn down the A/C because it will be too cold. However, this afternoon, they will be complaining about being hot. Any thoughts/ideas? Thanks.