Hey Poolman,
Your description of rapid cycling and poor performance is pretty much a text book description of low on refrigerant. My preffered way of finding leaks in R-134 cars is to charge them up and put in a little UV dye. Advise the customer that when the cooling performance falls off to come see me and we will finish diagnosing it. Unless you have a gross obvious leak it is kind of hit and miss to find the small ones with the electronic leak detectors and when you do find it with the dye you will have an obvious visual indicator. Unless you are set up to do this kind of work you are going to have to take it in to someone who is. Your other option is to feed it a can of 134 and see if it likes it and how long it takes for it to leak out again.

Mark