Do you have cruise control and does it work? It sounds like your speed sensor isn't working. If it's not that (cruise control works), it may be your idle air bypass valve.
I'd run the KOEO (quick test) and KOER (Engine running) tests to verify that all the sensors are hooked up properly and that the ECM can control RPMs via the idle air bypass valve.
When things are operating normally the ECM will see that you are moving based on feedback by the speed sensor. If the ECM knows you're moving and your foot is off the accelerator it will open the IAB valve to keep RPMs up so it won't stall.
One easy way to test this is to put the car in first and then accelerate until you hit about 1500 rpms (don't do this on a hill), hold that speed for a few seconds, and then take your foot off the accelerator. The ECM should be able to keep you rolling along without the engine stalling. If it surges a lot, it may just be a combination of a lumpy cam and the big throttle body. If it just stalls, I would guess that you don't have a working IAB valve or solenoid.
Pulling your codes will probably give you the best answer.
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351W 89 Mustang GT Convertible