Not very likely. That is because you stated that the car ran fine at higher speeds. Higher speeds would require more fuel so it should be worse if you had a fuel delivery problem.
I have written some long posts on the diagnosing and set-up of mixtures on your car. You might look through the archives on this.
Have you removed all vacuum from the distributor and verified the condition still exists. There are ways to get vacuum attached wrong that will give fluctuating timing in lower load conditions.
Bosch Master, ASE Master, L1
33 years MB technician