Uneven idle and sluggish acceleration

1987 Tempo GL with 2.3 4-cyl and auto trans. Odometer shows 78K, but who knows?

I have the two issues above. Both are intermittent. Sometimes the car runs just fine. Then with no warning it will lose power while driving along. Pressing on the gas just gives the sound of an open throttle with no acceleration. Usually, backing off will return it to normal, but at an idle. The worst was the other day when I had to run at just above idle and slowly get up to traffic speed. Even running at 45 mph, I could not add throttle and get any acceleration. Usually, though, I can throw it in neutral, hit the pedal until it starts to rev up, fairly shortly get back to normal operation, and run just fine for a while, up and down the gears and speed without a hitch. It might go a few days with no problem, then back to the acting up. It starts easily.

The idle often drops off. It will run normally for about 10 seconds, then drop off for a bit and regain normal idle speed, again for about ten seconds and dropping off again.

I have noticed that in gear and with the brakes on, the idle evens out.

I should say the engine has been out of the car twice, once to change the flywheel and once to swap transmissions. I have changed a lot of parts in the process, over three years or so. This includes the fuel pump, the injector, the usual tune-up parts. I have tracked down all the vacuum leaks I can find.

I had Autozone check the trouble codes. First time showed DC Motor not turning, and EEC failing. Second check, after I fixed a vacuum leak, showed no codes. The car ran fine for ten miles or so, then started acting up again. Not so bad as before, but it is not fixed and I don't want to end up on the side of the road somewhere.

Any ideas?