I had the same mysterious bucking problem and tried many of the fixes listed on this thread to no avail. I tested the TPS and found that it wasn't working correctly around 2200 RPM, so I got it replaced and it worked great for a while, then the bucking came back, at first softly, then noticeably stronger. We took the truck back to the dealership who installed the TPS and they had it for three days scratching their heads until they found the problem.
It was the wire that leaves the TPS and goes to the ECU. It travels in a wiring harness under the throttle body towards the front of the vehicle, then turns downwards and goes under the intake manifold and back to the firewall. What had happened was the TPS wire insulation broke down and was grounding against the intake manifold. This over time "burned" a patch on the TPS sensor at the most common RPM we drove at, and eventually that area was so damaged that it didn't send information back the ECU, causing the bucking problem.
The dealer simply spliced a new wire and ran it directly to the firewall, outside of the harness it usually goes through. We've driven it for two months now with no problem at all!!!
To all of our credit, they had their top service tech (reportedly one of the top techs on the whole west coast) on our Tracker for three days before he tracked it down, it was even stumping him!
I know this thread is kind of old, but I promised myself that I would post this information in the hopes that it helps someone out. We lived with this problem for eight months, and it truly sucked. Glad to have the Tracker running strong again.
Cheers,
DJAthens