Squirrly is what I call this. I own a '97 E320 and it had exactly this
subtle steering behavior.
IMHO the tires on both our cars are too large and magnify such problems.
OK, here's why.
I found that the front wheels' toe in/out caused the squirrly steering.
As one drives on a local street, say at 25 mph, the car seems to favor
first one side then the other. IMHO this is caused by rises and dips in
the road acting on first one wheel then the other. If the front toe-in
or toe-out exceeds specification the car will self steer because the
wheels lack parallelism. Visualize too much toe-in - the leading edges
of the front wheels closer together than their rear edges (spec. is
about 1/8") - and a rise in the roadway on one side only, the wheel that
now must go up will steer the car in its direction vs. the direction the
other, now less loaded, wheel is pointed.
I check the front alignment thus: Exactly center the steering wheel and
drive into the garage or other flat surface stopping the car with the
parking brake. The steering wheel is dead center so each front wheel
also should be too. The body overhangs the wheels so one needs to use
spacers to compare the front to the rear wheel. I hold a 2" wide bubble
level as a shim (not for its bubble but for its consistent width) on the
front alloy wheel and sight down to the rear wheel against which I
placed a Chevron Techron plastic bottle (not that that's special but it
gives a 2.5" space from the wheel). Using this simple method I've found
one wheel was toed out and caused the squirrly steering. One can
correctly calculate the exact width of the rear spacer using high school
geometry, adding the front spacer's width and adjusting for the car's
front vs. rear track widths. But for a quick check these figures will
Hope this solves the problem.