Help - Intermittent momentary engine missing 500sl

[BAckert1]

I could really use some help with this one. I own a 93 500sl with ~84,000 miles. Last summer and this spring I have noticed a very slight missing that occurs is noticiable at any constant engine RPM or road speed. The missing is very short lived, just feels like a little tug backwards. Most of the time it doesn't even show up on the tach - other times I believe I can catch a fleeting every so slight dip in RPMs. It can occur when the car is hot or when cold. It can occur when the weather is hot or cold. It can occur if the gas tank is full or near empty. It can occur if the car is idling or moving at 90mph. It can occur some days and not at all on other days. It can occur every few seconds or every few minutes. The only things I can tell is that when it occurs, the engine speed is steady (never notice anything during accelration) , and when it occurs it appears the missing seems to be consistent in time from miss to miss although that miss interval may be different different days. Any ideas? If it had any dependence on RPM, temp, car speed, gear, tank level, etc it would be easier to diagnose. Seems to irregular to be fuel filter, injector, fuel tank pressure, fule distributor, etc....... I worry about impending failure of some component but don't have an idea of where to start.

Thanks,

Todd