AC intermittent

I have a 2000 Ford Focus SE. My AC seems to be intermittent. Sometimes it works real good and sometimes the compressor goes on and off. Is this a sign it needs a charge? Not that I would do it but
where do you connect the charger?
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Yeah, good luck finding the low pressure fill valve. I Looked for quite some time and came up with nothing (although by only the greatest stretch of imagination could you call me a mechanic) - I took it to a local reputable service station and they said that I was stupid to try to fill it myself, that if I over filled it or underfilled it, I could do more harm than good and ruin my compressor. I exclaimed that the filler bottle came with a complimentary pressure guage (complete with a red, yellow, and green zone!), to which they retorted that it was inaccurate and a sure way to screw my car over.
Would someone please comment on this?
Thanks!
JD

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