l had a 2.5 L from a '04 STi put into my '98 2.5RS.Been fine until the last couple months.Ever so often, at least once a day, the engine shuts off.It acts as though its not getting any fuel.Sometimes while driving down the road the RPMs will drop then the engine will shut off.Please help.

l agree with your diagnosis.
l am not a mechanic but it sounds like fuel.
It could possibly be that the in-tank fuel pump and/or filter assembly coul be on it is last legs. This usually happens before the pump gives up the ghost.
Failing that, check the injectors and external fuel filter.

what modifications have you done to the car?