peculiar problem with 93 100S

[James Dean <qwe@wam.umd.edu>]

Hello.

I have a 93 100S with a peculiar problem. The check engine light comes on
sometimes when I drive for a prolonged time keeping the revs below 4000
rpm. 

When I drive briskly. . .shifting above 4000 rpm and keeping the revs up.
The light does not come on. 

I have done the following mods to my car. Added a Racing Sports Akimoto
cone airfilter, Remus Exhaust, and hi-flo cats. Could the increased air
intake and exhaust be messing up the computer?

When does the check engine light come on usually? 

thanks in advance

Seung Lee (I wish I was James Dean driving a silver Boxster)

93 100S