Help please!!!

I've searched these forums for possible causes, but nothing has worked so far.
This past week my Tracker started Running rough on my way to work.  It would idle great but as soon as I let out the clutch and put it under power it would cough and sputter like it was cutting out and then starting again.  This happens between 1500 and 2000 RPM's.  When I shift into second, the power loss jerks the Tracker and makes a passenger feel as if I'm just learning how to drive a manual.  I recently started running it on the freeway to/from work since selling my other car and I'm not sure if the high speeds has something to do with why it started running like this lately.  So far I've changed the spark plugs (NGK PFR6) and fuel filter.  The Cap and Rotor look normal, and the wires aren't that old, maybe a couple years at most.  I was messing around with the timing too, trying to tweek it and get rid of that supttering spot, but no luck there either.  I'm wondering if there's something else that is causing it like a bad sensor or something, but there hasn't been any engine codes that have came up either. 

Have any of you experenced a similar problem?  Any ideas on what might be causing it?

I would start with a complete tune up! Just cause the cap and rotor look fine doesn't mean they are! And a couple of years on the wires I would say change them also! Thats where I would start!

I'd start with a compression test test to make sure that the motor is sound. Then do the tune up and fit a new fuel filter.

Try adjusting the valves and ignition timing. If one of the valves is staying open slightly it will make it run bad.

Try adjusting the valves and ignition timing. If one of the valves is staying open slightly it will make it run bad.

I'd start with a compression test test to make sure that the motor is sound. Then do the tune up and fit a new fuel filter.

In response to your replies! Wouldn't you think it would run bad all the time if it was one of these problems! It seems to be more of a under load issue!

yeah that's what I'm thinking too.  It only does the hesitation between 1500 and 2000 RPM's, i.e. right when you give it gas for take off or an upshift.  Anything above 2000 RPM's it runs fine, Lately I've been reving it higher and shifting at higher RPM's just so I wont hit that hesitation spot.  I just changed the fuel filter and I'm running a bottle of STP fuel injector cleaner, but there's not any improvement on the problem.  IDK what else to do.