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You really should get a gauge set and read the high and low side pressures to know what is really going on. With that said, the low side has a pressure sensor that shuts off the pump when it gets lower then about 25 to 27 psi. That is why your pump is cutting in and out; the low side pressure is too low.
Most likely you will need to add more so that the low side pressure stays a little higher and the pump runs all the time (or at least most of the time). You should also monitor the high side when you do this so the high side pressure doesn't get too high. If the high side pressure gets too high it will destroy the pump.
If you don't have the money to get a gauge set, or have a professional work on it, and want to risk blowing the pump out add just enough to keep the low side pressure around 25-30 psi while the engine is running at 1,200 to 1,500 rpm.
I am not an AC professional, but I have worked on a few fords.