Hyundai Repair: transmission 2000 hyundai elantra gls fwd automatic transmission, hyundai elantra, 2000 hyundai elantra

Question
i had just changed my radiator and i went home and left in the morning for work and the transmission just stopped working in motion the car didn't die it stayed running i tried to shift it into park then all other gears after to see if it would move the engine just reved up and didn't move at all i had just done a complete transmission flush a week before due to the fluid being a dark blackish gray color i was told i needed a map sensor for the car but i never got around to it it was hard shifting alot and when i hit over 3750 rpms the engine wouldn't shift right and it would overrev until i let off the gas and let it catch and it would go fine unless i exceeded that amount of rpm so i put some lucas transmission cleaner and lucas fuel injector cleaner in the car after i did the oil/transmission fluid changes what do you think is the problem?  

Answer
I think most likely your transmission needs to be replaced or overhauled.  The symptoms you describe occurring before this event support that diagnosis.  Dark gray fluid, hard shifting, and overrevving are all symptoms of a transmission with internal problems.  

Since you've recently replaced your radiator, first check to make sure your transmission fluid is at the proper level.  It's possible there was a problem with one of the cooler lines and enough fluid has leaked out to cause this issue.  But even if that's all you need now, you should prepare for a major transmission repair in the near future.  Transmission cleaners and fluid additives are most often only temporary fixes for serious problems.