Such A Quirky Tahoe

Here is a strange occurance... Just replaced (leaking) high side fitting (nipple) on my '96 Tahoe with A/C... I had been losing my R-134A fluid, and saved myself about $350.00 by locating the leak, and replacing this fitting..cost $8.18 at chevy dealer..

Now here is the quirky problem... Starting the Tahoe with the A/C off, and afterward turn on the A/C it gets real cool..fast... If I leave the A/C on, and start the Tahoe, the A/C is not as cold.. medium cool...
I put a meter gauge on the low side withe A/C off. The needle fluctuates from "normal fluid charged capacity to "overcharged" capacity, and back and forth.. When I leave the A/C "on", and start the Tahoe, and put the meter gauge on the low side, I get no fluctuation at all.. just a "normal" charged..

Any idea the cause... and solve the minor problem?

You need to get " real" gages on this and look at high and low side pressures for a proper diagnosis.