I'm not really sure what's happening with my 87 GT. From 0-65 it behaves normally, but 65-80 it acts like it wants to die, like there isn't enough fuel getting to the motor. But, after 80 it's fine, even up to really high speeds, I had it at 102 and it was running fine. Also, the service engine light comes on and the engine slows down, then it speeds back up and the light goes out, but it keeps doing this over and over until i get below 65 or above 80. At first I thought it was the alternator, not making enough electricity to run the fuel system & the accessories at highway speeds (I eliminated that theory (I shut off everything in the cabin, still did it)), then I thought something was clogging the fuel lines, but I remembered I had used Seafoam (Again theory busted).
Next I thought that fuel pump could be going bad, but that theory is out the window, if the pump were going bad, it would do that at ALL speeds, right? (Correct me if I'm wrong.)
I'm just throwing out one of the ideas I had, but I think that when the car was new, someone tried to install a fuel cutoff and it didn't work right so they just left it on. Now, it somehow started working and is trying to cut the fuel off at 65. Again, just something I've considered when trying to fix this problem.
If the trouble is still happening, it would be a good idea to see what the fuel pressure is doing when this happens. If something was causing the fuel pressure to drop, and I have no idea what that something could be, but, if it were, you could be leaning out and getting the check engine light for that reason.
Two people are needed for this. Make sure you have no fuel leaks before taking off with the gauge attached. You can route it out the side engine grate (removed) and tape the gauge to the back windscreen.