I've had the same issue with my 156. RPM varied like crazy during the traffic and it was really annoying. We checked out few issues;
- ignition coils
- throttle body
- MAF
However, as non of them were faulty, my mechanic re-tensioned a spring which was inside the throttle body. After that the problem was resolved. However, couple of months later, MAF also gave problems alongside with the lambda sensor again giving slight RPM drops. Detached the lambda and running with another MAF now and the issue is completely gone.
So my guess is check the throttle body with regulators. Fuel pump can be an issue considering the quality of fuel we have. Check the MAF and coils/plugs.
Good Luck!