they make a good point with the fuel pump answer, but my 89 formula 350 was doing the same thing, and it turned out that 3 injectors were going/gone bad on the car. sometimes it worked, sometimes is stumbled, then it sat for a day and was fine for a bit. and yes, at a certain rpm range the car seemed like it was all messed up. i don't remember which numbers they were that had gone, but i had a local shop do diagnostics on it, because like you i didn't want to have to do the feul pump unless absolutly sure about it. just my 2 cents

 

ignore this post, hit the wrong button guys, sorry