i had the same problem with my 05 evo 8. I had an aftermarket oil gauge that was showing everything was fine, so i just had the dealer check the stock oil pressure sensor, and it just needed to be cleaned and then it was fine, they said if it happened again they would replace the sensor.

 

anyone knows where or how to clean the oil sensor?

 

Its on the timing belt side of the engine, near the oil filter. It is shaped like a "bell" sort of. I seriously doubt "cleaning" it will help.