I'm having a problem, and i've run out of ideas for what it could be. Everytime I hit around 1900rpm the car hesitates, almost like a slight miss, and it continues to about 2800rpm. After 3000rpm no problems at all. It happens in every gear, if I throttle it slowly or quickly. The exhaust also has a weird higher pitched tone (even at idle). It's a 98 with a 99 motor, 98 DIS and fuel system. It's run fine for over a year with this setup.

Here's what i've checked:

Ignition working at all rpm (spark)
Injectors are clean/working
all vacuum lines connected/no leaks
all sensors/electrical connected
NO CHECK ENGINE LIGHT

I dont know what else to check. I've thought about the cats...maybe one is clogged. Would that cause a problem at these specific rpms? I havent checked compression yet, mainly because im afraid to.

Any ideas??????