What is the CFM on your AC? I am running a IR AC that sounds like yours, It is a 17CFM. It runs great with our 1650 and we can run die grinder at the same time. It sounds like you may have a motor or pump problem. does the pump spin easily when it shuts down? That might indicate a motor problem, if the pump is tight then it might be a pump. Another thing to consider is the unloader, does overload trip when it tries to start or when it is running? If the unloader is not dumping the line pressure then the motor will have to overcome the hi-PSI to start, which puts a lot of stress on the start capacitors.
Hope this helps, good luck!