Please realize that without a check engine light, everything becomes a guessing game. From experience, I have found the rubber boots on the coil packs (that go in the plugs) break down and allow the coil to short through them. It is hard to find because the only evidence is a small light gray spot on the boot itself. The interesting thing is it is more prominent when there is a load on the engine. When you shift at a higher speed, the load changes and demands more from the engine.
At this point, that is a theory, but it's something you could check.
Let me know.
Wednesday, December 23rd, 2009 AT 10:26 AM