Weird air con problem with turbo 280

After I run for 8-10 miles with the A/C on the vehicle starts to surge. Either losing ignition or fuel too lean/rich. Turn off A/C immediatley starts running fine. Turn a/c on vehicle surges. I can turn the thermostat up to high and the surging continues but that should eliminate anything electrical with the compressor and it isn't seizing up. Vehicle runs normally when first using a/c needs certain amount of run time for problem to arise. I looked at all the vacuum hoses I could find and they appear to be sound. Car is an 81 turbo with a 5speed conversion, 138K on the clock. Runs really nice other than the a/c problem. Believe injectors/fuel pump okay cause I can get it going pretty fast through the gears with no problem runs strong. I'm baffled as the only link I know of to a/c is the idle boost when you kick it in.

Roger