[FearlessFoe]

hmm.. Was overdrive on? You might want to check your torque converter and look through that.. The way it works is from fluid being forced to the sides when it spins faster, which creates friction between plates. Sometimes if theres not enough fluid, it won't grip. But that doesn't seem like that'd be it because you are still moving in first and second.. Do you know how old your transmission fluid is? It might need to be changed..