Air Conditioner - serviced, worked for 30 minutes then quit

My A/C has been out for about 18 months, living on the remote "wet coast" of Vancouver Island it hasn't been too bad, but anywhere east of the fog-bank, and I miss it!
I took it to the service shop today (100 km away), who topped it up, and low and behold it started working! They said they could not find a leak on their pressure testing, but added some ultra-violet die in case it was.
Within 30 minutes of heading home the a/c quit, and the a/c light began flashing on and off. I assumed that it had lost its charge, and continued home for another 30 minutes, where upon I tried it again and discovered it was working again!!!
Not having access to a black-light to test for ultra-violet traces from a leak, I am wondering if there could be something else that caused the flashing light, before I drive another 100 km back to the service station?
I read somewhere that a dirty interior filter may be the cause? or possible a slipping belt? or even a faulty thermostat?
Anyone have any suggestions for me to check out, please...