I have a 1991 red miata, when driving it at first it is fine. After about 30 minutes though, the car acts strange. The car begins having a lack of power and the rpm's will begin climbing when shifting gears but the car will not go fast. I am not sure if the clutch is going out, or if it has something to do with the camshaft. Any advise......

 

Does kind of sound like clutch slipping ebarragan. Quick way to find out. Drive it for the 30 minutes so problem begins to happen. Stop car and rev engine to 2500 rpm, shift into 3rd gear and just let the clutch pedal out very quickly. If engine stalls immediately then clutch is gripping good. If RPM slowly drops then clutch is slipping. Slipping clutch will also give off a really nasty odor.