2003 s2000, 140k miles

First, some background:
Engine was replaced around 20k miles ago, spun bearing from low oil. I have had a cel for the air pump for maybe the last 5 years.

Current engine has been okay up until recently. Doesn't make any knocking sounds, so I doubt it's mechanical.

Car runs great up to 4000 rpms, but beyond that it starts to buck a little, like a temporary loss of power. This only seems to be an issue under load, as reving above 4K in neutral appears to sound okay. Furthermore, the issue is intermittent, as most normal driving is okay. It usually only happens on the highway beyond 65mph (4K rpms in 6th).

I've tried cleaning the map sensor, but it only helps temporarily.

I have a set of spark plugs I'm going to put in tomorrow, but I'm concerned it may be something more. I don't think it's the ignition coils.

One concern is it may be valve retainers. Anyone in San Antonio have experience switching ap1 to ap2 retainers? I looked over a diy, but the task is well above my experience.

 

Okay, thanks for looking!

 

When you pull old spark plugs look to see that they all look consistent. Any outlier may point to a problem with something on that cylinder. If it's bucking then you most likely will get a CEL registered. Check your computer again. Check air filter as well. Reason it shows up on highway is typical of misfire, as you move through RPM slower on highway so any issue will present itself easier and over a more recognizable period of time.

 

listen to ian, he knows whats up. wondering is IAC valve needs to be cleaned, sounds like your map sensor is acting up some