stumped with check engine light

Now where to check?

When i consistenly run 3k rpms (about 15 minutes or so) and let off the gas the check engine light comes on. It doesn't come on in city driving; I'm assuming the stop and go. I know the light is usually emission related 90% of the time. It is not a 'hard set' as the light always goes off when the car is turned off and restarted. I've taken it to the mechanic but he couldn't get it to "flash" for the problem (light was on and I left the car running). I cleaned the egr valve and tubing running to the intake. It was pretty plugged up. It def runs smother, but the light still comes on.

When I had it emission tested a few months ago; they said they had to do it 3 times because they couldn't get a reading the first 2 times. Thats why I was thinking maybe the egr. It passed btw.

I have a 91 with a jdm engine and transmission installed (about 600 miles since it has been in the car). I have an aftermarket air intake (cheapy off of ebay; I bought it for my 94 but the tubes weren't the right size, but works in the 91), I was thinking this might be the culprit???, but the oem air box is 500miles away in my storage building....

Any suggestions would be GREATLY appreciated!!!!