Question about A/C refrigerant fluctuation

[jase]

While trying to figure out why I had exhaust fumes in the car, I noticed that the A/C has begun to not cool as it once did. I have a 2000 V6 3.8L. I replaced the A/C clutch about 5 years ago. That is all that I have done to the A/C system and I am the only owner of this car. I've had it since 2000 and have almost 300,000 miles on it. I thought it may need a refrigerant recharge and bought one of those all-in-one kits from Auto Zone with a built on gauge on it. According to the gauge, it didn't need refrigerant once it got to it's maximum point while the clutch was engaged. When the clutch disengaged, the dial went down really low, but came back up to its normal operating point once the clutch engaged again. Is this huge fluctuation normal for A/C systems? Just curious because I know nothing about A/C systems. I still don't know why it's not blowing as cool as it used to.