i think its my transmission.

[Mr 5 0]

Help us out with more info;

What year Mustang? (I assume it's a 5.0).

Is it an AOD or T-5 (manual).

Did you go from 15" wheels to 16"?

The cruise control glitches could well be electrical and have nothing to do with the transmission.

Going to 16" wheels from 15" would change the RPM's (downward) at a given speed.

If it's a manual (T-5), the clutch could be slipping at higher RPM's. If an AOD, the fifth gear band may be going out.

Either way, change the transmission fluid immediately and see what happens; I strongly recommend synthetic trans fluid.