I was having a problem like this but only in first gear. At around 5k RPM my engine seemed to stall a little and didn't have that nice linear increase in RPM in the power band. I checked the engine and the o-ring that holds the MAF to the throttle body (I guess that is what it is) wasn't very tight. I tightened this but it didn't seem to fix the problem. I was kind of low on fuel (even though my low fuel light never came on) but I decided to go fill up on some premium gas (usually I use only regular). That seemed to fix the problem but is this totally coincendental (however you spell it)?
