Lately my 1990 300E has been running fine for about 15 minutes, then starting to miss and sputter slightly at low RPMs. When waiting at stoplights, it runs rough and seems about to stall (but it never does). When starting to move forward, there is sputtering and loss of power on acceleration until the engine RPMs get into the upper range. I find it odd that this only happens when the engine is warm, but not when I've just started it.

Any ideas?

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Vienna, Virginia
1998 E320