take it to a shop, it could be something as simple as a sensor. a similar thing happened to my brothers ford ranger, his OD light would flash cruise control wouldnt work, when it would shift it clunks and throws you about inside the cab, RPM's would skyrocket and his speedometer stopped working. took it to the shop, spent 120 dollars and EVERYTHING i mean everything was perfect, everything i said was all fixed by this one stupid little sensor, so i would get it checked.
