There really is no other place, other than the headgasket, for oil to get into the reservoir. However, the radiator, if silently broken within, can allow transmission fluid to mix with your coolant, and vice versa. So, if coolant and tranny are mixing in one place, it seems rather certain they would be mixing in the other. Check your transmission fluid. If it looks like strawberry milk, you need a radiator, and a few transmission fluid changes, all with some sort of transmission conditioner.

As far as power goes, if what is stated above is happening, obviously the transmission won't downshift properly. If this isn't the case, you really should have a check engine light. Other than that 90K was tune-up time. If you have the original cap and rotor, these would be suspect. Plug wires should be fine, though many of my customers want them changed by 90K.

Chris