Hi,
Change out the coil. I had the same problems with my '87 300E. My mechanic tried all sorts of things over a three month period but to no avail (he didn't charge either). Finally mentioned problem to my son who owns a Passat. Same thing happened to him. It was the coil. Ran fine at high speed but was flaky at low speeds. Go figure.
Anziani

'87 300E 200K
'95 E420 110K