Compressor cycling frequently

I have a 71 Dart. I did a R134 a/c conversion on an original a/c car. The a/c compressor cycles about every 30 seconds. I had it checked by a few shops they drained the refrigerant checked for leaks and refilled it and I have the same issue. What should I be checking next? I also jumped the dryer switch and the issue continued. Any help would be greatly appreciated I’m stumped