I spoke with a technician about this before and his recommendation was that the oxygen sensor could be the problem. I apologize for the length of this, but wanted to make sure to get all of the information here. You probably do have a faulty oxygen sensor. With 91000 miles on your car, it would be good practice to replace it anyway. If your problem persists, then scan the computer for fault codes.
Oxygen sensors go out of range at times, but still work.
I also notice the tach jumping between 500 - 1000 rpm when in drive. The car has a 4 speed automatic and it has been tuned up about 30,000 miles ago. The MAF sensor has been replaced, but I'm thinking the TPS sensor needs replacing. My problem started about 2 months ago and I have not been able to resolve it. Once running it will vary in power when downshifting, while coming to a stop it will suddenly lurch forward.