In the fall, after I installed the winter tires, I noticed that the car would downshift while in cruise control mode on sections of the four lane highway and it did not do this with the summer tires. I just thought that it was the higher rolling resistance of the winter tires (although I did not notice this the winter before). But since that time the car seems to be getting nosier, is now downshifting into second on portions of the four laner, seems to have less power, and last night on the highway it lurched sharply three times over a period of one hour at 120 km/h as if a cylinder or two suddenly went off line and then came back on. The lurching felt like gear lash on a car with a manual transmission. This lurching also reminded me of a K-car we had after it developed a head gasket leak. Last time I checked, all the fluids were clean, no cross contamination. Couple of things: the rear muffler was very badly corroded and I patched it last summer but now I wander if the guts have collapsed and are causing back pressure, and I noticed a muffler sound around the cat where there was none before, so I wander if I am developing a hole in that area or the joint is starting to leak.

Any help in troubleshooting is greatly appreciated.

92-960, 200 000 km