Strange Issue...

What codes do you get?

The computer should have them stored if I'm not mistaken. Either way, when an issue pops up, it's always a good idea to go for the codes first! I've learned a lot from this site, and that's one of the biggest things. You could have a problem that you think is something big, when in reality could be a bad o2, or dirty MAF, or just in need of a good tune-up.

Check the codes, and I'm sure the guys here will help out. However I had a 'no power gain' issue twice since owning my car. Once was caused by a very dirty MAF, and the other was because my spark plugs needed replacing, and they were platinums which, when they get old, cause breakup in higher rpms apparently.

Jeff is correct. The EEC can even store codes when the CEL has never illuminated. Thus anytime an issue arises, pull codes whether the CEL has been flashing, on constantly, or not at all.

You're likely to find a lean A/F or EGR issue but there is no point in guessing when the codes will tell just where to concentrate.

Good luck.