You didn't give your 900 model or year. For 700/900's there was a change starting with the 1992 model year (first year of R134?). In addition to the low pressure sensor (pressostat) on the receiver/dryer there's also a high pressure sensor (violet color) on the passenger side, bottom end rail of the condenser. It's visible through the knockout in the bumper skirt. Make sure the connection is clean, snug and the wires aren't damaged. It's wired in series with the low pressure sensor both to activate the compressor and to give the ECU a signal that the compressor is (supposedly) operating (to raise the idle slightly). If you happen to have a 960 with ACC (automatic climate control) then there's also block, cabin and exterior ambient temp sensors plus a sun sensor all feeding an electronic control module which also turns the compressor on and off.

I'm not an A/C expert in the least, but I do hear a lot of comments that there's a tendency to overcharge R134a systems -more is not better and of course less is not good. If you put in slightly too little R134 then the low pressure sensor may not kick in to start your compressor when the engine is cold, but as the engine warms it may be getting just enough pressure to close the pressostat -short cycling the compressor. Conversely (at least it makes sense to me), if you put in too much R134a then the high pressure sensor could start kicking in and shutting down your compressor prematurely, especially once the engine warmed up -again short cycling the compressor. If I read your post right, the former might be your case -too little R134.

For a DIY A/C system overhaul, unless you were careful about measuring refrigerant amounts and capable of measuring pressure then I personally wouldn't jump to conclusions that it's a faulty pressostat.

Here's an old thread regarding A/C system troubleshooting you may find useful. Note that it's for an early 850 with freon, so the specified values for your 900 R134a system will be different.
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day

Dear larrytay88,

May this find you well. The pressostat is mounted on the receiver/dryer. This is the round, aluminum, can-like unit, mounted on the strut tower, just in front of the firewall on the passenger side of the engine bay (USA/Canada models). It is connected to several 1/2"-diameter aluminum tubes.

The pressostat is at the 2 o'clock position on the receiver/dryer and has a two-wire connector (blue plastic on the 1993 940).

Hope this helps.

Yours faithfully,

spook