Erractic hiccup at idle in 300CE

[endeavor]

I have a 1990 300CE and generally when it is hot outside, the car at idle under load has an erractic hiccup and actually shakes the engine, 104.980 engine -24 valve.
This hiccup or miss comes and goes. Putting 93 octane or 89 or 83 does not make any difference.
I have replaced all the ignition components, new plugs, wires, fuel components such as distributor, injectors, fuel pumps. I also checked for all vacuum leaks and fixed all of them.
The thing I have not checked is the fuel pressure regulator, fuel pressure, throttle valve switch or the plate.
What am I not sure is it whether the electrical or fuel related?
I am quite handy, so if some experts on MB can help me, would be great.
Any advice would be great.
bshah@satx.rr.com