Lots of things cause surges, but surges also mean different things to different people. I associate the word surge with performance problems that cause smooth changes in power. The other word used is misfire. Misfire is a single or multiple cylinder drop in power that I associate with vibration.
Surges usually are fuel or timing related - things that affect everything but not bad enough to cause misfires. Single cylonder misfires are usually caused by plus, plug wire resistors or single injectors. Vacuum leaks can cause single cylinder leaks at idle that go away as things move faster.
One common idle misfire on both 420s and 560s is the vacuum line that goes to the transmission modulator. The line attaches to the lower manifold at the back of the motor right above the "HOT" crossover pipe for exhaust. The heat does in the small rubber connector.
The reason I mention this is that I key off the fact that you say it doesn't happen cold. The main difference cold that helps cars run is mixture. Everything else works against cold running. Ignition should have a worse time cold. But the car will be running rich enough on cold enrichment to get by a small single cylinder or two misfire.
If none of this help, describe the conditions the problem exists. Engine speed? Engine temp? Throttle position? A/C 0n/off? Any other repetative characteristics? How do you feel the problem?
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician