Did you have the conversion done by a reputable shop, or someone that has experience with doing conversions? There are more steps to doing a conversion correct than just changing the refrigerant. All of the systems o-rings should be changed along with a few other parts and flushing, changing or adjusting the pressure switch and other things. If it has been working right until now and it works off and on, I assume to be on for a few seconds then off for a few then back on it's most likely low. If it is low there is a reason, cars don't just lose refrigerant. If there is not pattern to it's operation then yeah the switch could be bad. Not sure where they are on the Volvo's (my original r-12 system works, so I have not messed with it)but they are usually about $20 and screw on and off just liek the service ports fittings. Make sure you get one that is for r-134a, they operate at different pressures.