I have a 1988 RX7 Turbo that has been sitting for a lot of years without being driven. When I started it originally it idled fine but had misfire issues. So I pulled the fuel tank and cleaned it out, blew out the fuel lines, replaced the fuel filter, fuel pump and added premium fuel to tank. When I started the engine it idles fine and runs great until it is put under a load. Shortly after you take off it will start to misfires. If I put it in neutral it will go right back to an even idle and runs well but will misfire if I put it under much of a load. It seems to run ok at low RPM's under a very small load but will immediately start to severely misfire and falls on its face under a heavier load. If I turn the key off to energize the fuel system to pressurize the fuel lines it will pull hard for a short time and run perfectly but soon will misfire acting like it is fuel starved.
Can anyone tell me what I should look at next to try to diagnose the engine misfire or possible fuel starvation issue.

 

I cleaned out a rusty tank, threw in a new fuel pump, new fuel filter, and it ran ok for a short while until, like your car, under load it would struggle and misfire.

I pulled the pump and the sock was totally clogged and dirty with rusty bits still. I've cleaned it twice since, and it keeps happening. I'm now on the hunt for a rust free tank.

I would go back and check your fuel system one more time to rule that out first.