Have a 2001 monte carlo that seems to be surging in and out of overdrive. This doesn't happen all the time but when it does I can hit the pedel to downshift and is fine. No trouble lights on or anything. Only seems to do it in overdrive.

I would take that to the tranny place! It could be a variety of things!

check cat converter was having same problem recently

had the same problem, had an exhaust manifold leak,had the manifold repaired and the problem was gone, check for any exhaust leaks before o2 sensor

same prob here with a donut gasket on the manifold leaking. Isn't that wierd, though?? Exhaust controls transaxle......