Distraught

Hi All, Firstly I appoligize if this is a repost but I'm slowly losing my mind. Neighbour has a 1988 GL wagon, 5 speed 4wd. Did a tune up, replaced the plugs, leads, cap and rotor. Did the fuel filter, air filter and replaced the fuel pump that had died. The issue and what I have done so far. Hesitates under load badly. I have replaced the fuel pressure regulator, idle air control valve with a known good part, throttle body, all the venting tubes. The only way I can get the car to run properly is to clamp the return fuel line to tank. I have checked the timing and it is right on now that I have fixed all the vacuum leaks. The only thing I can hear that had me slightly baffled is the fuel pressure clicking on and off every 5 seconds. Was hoping to get some advice before I went out and spent 200 dollars on new fuel pressure dampers