Help with car stuttering/sputtering

[twofortySXY]

I have a fairly stock 93 240 coupe with injen intake, DC headers, ACT clutch, and short shift. After installing all of these I never had any problems. Maybe about 6 months ago my engine overheated and the head warped which had to be resurfaced. This summer I started noticing that sometimes (usually after my car warms up, and moreso when it's warmer out) under acceleration my car stutters and the harder I push on the pedal harder the stutter gets worse. It's not consistent though, it could do this one second and not the next.
Also, sometimes if I floor the pedal, it will stutter at first and then come out of it in one of two ways:
1. accelerates normally
2. engine sounds as if it is accelerating normally, but is significantly less than normal

Because of the second outcome I thought that all this could be to the clutch slipping. I thought that since the engine revs normal but is slower I assumed that had to be it because in a certain gear at a certain RPM the car would have to go a certain speed, but i have a feeling I'm mistaken. Does a loss in power result in outcome #2 above?
I'm looking to do a rallycross in 2 weeks so I want to get this fixed as soon as possible. Usually if I gradually push the accelerator pedal non of these symptoms occur. Also, if I push in the clutch or am in neutral and rev the engine quickly the stuttering NEVER happens; only when the car is under load. I've been on numerous forums where people have similar problems, but there doesn't seem to be one clear answer, and it seems everyone has a hard time diagnosing this. Any help would be greatly appreciated.
One other this is I had my radiator placed a few months ago, and once in awhile my engine temp meter will spike quickly and then go back to normal.

[EnjoiNismo]

Sounds like a fuel issue.
I had something similar back in the day, it would sputtering sometimes then go back to normal.
Does it sound like its running on 3 cylinders for just a moment? One of my injectors finally went out.

A friend also had the same problem turned out to be his fuel pump was going bad.

As for the radiator, sounds like you just have a air bubble in the system.