My wife has a 2001 Elantra with 1.8 litre motor. A few times, it seems the transmission
is not shifting up, stuck in second or third so the engine RPM is much higher than normal.
But the next time you start it, it behaves as normal. It had a transmission flush recently.
If the MAP sensor were kaput, I expect the engine would also run quite poorly. I have learned that the Elantra has input and output speed sensors. Can these become intermittent, or do they die for good?
It has done 162,000 km and the original owner was an old lady who provided all the records showing it was regularly serviced at dealer workshops.

 

Please try the LUCAS "transmission fix" before you do any costly reairs - help in some cases with stuck valves in your tranny - after some time... good luck.!!!