1992 miata hesitation under 4200rpm
 

I just bought a used '92 Miata with 170,000 miles as a project. It's a 5 speed.Themotor seems to run very nice. No tapping or rattling, smooth acceleration in neutral. Sounds great.Here's the problem: When I'macceleratingthrough the gears the car seems to hesitate and and surge until I reach 4200rpm and then it pulls like a horse. During acceleration the engine sounds like it's firing properly and the rpm's increase evenly butthe ride is jerky and uneven. I changed the spark-plugs and wires. I replaced the fuel filter. I've run a tank of injector cleaner and I even sprayed the crap out of the throttle-body with throttle-body cleaner (but did not remove it, just sprayed the paddle anddown its throat). I've read a bunch of opinions on similar problems but most either don't hold water or I already tried them. I don't think it'sa weakfuel pump or it would get worse at higher rpm's... right? The O2 sensor has been suggested but I believe I would have a "trouble" light letting me know... right? The clutch is a little high but it seems to grab like it should. Anyone know what my problem is??? Besides not being smart enough to figure this out?! Thanks in advance!