99 Accord issue while accelerating, and near 4000 RPMs.

I have a 1999 Honda accord, 2 dr. coupe, manual transmission. The car has 143,500 miles. Recently the check engine light came on (what a surprise), and most of the time it's nothing major, but I recently notice that as I was driving to get onto a highway for instance, whether it was 1,2, or 3rd gear, as I was around 4000 RPMs, all of a sudden for a split second the car felt as if I down shifted, and it was slow down. I noticed this while in 4 gear also, and can't remember if this happened in 5th gear. This car has been working very well up to this point. I do regular maint., the timing belt/water pump has been changed at 80K. It doesn't happen all the time either. I can't remember but the check engine can come on if you need motor oil right? I did check the oil today, and it was low, so I did fill it up, the light is still on. Has anyone had this acceleration problem? Thanks.