Well, I don't know if you've checked this but have you considered your cold start valve? My 560 didn't need it, but when I got her I decided that I'd replace every valve, vac line, and all of the little odds and ends under the hood to get a squeaky clean look whether the part was working fine or not (I'd just put the old part in the new box and file it on my "560" part shelf
This was my first 126 (as I'm a 115 fan) and I would do a little research on the 126 parts as I replaced them... the cold start valve is one of the components that helps with the immediate startup and idle of the car when its cold... You may also want to check out your o2 sensor although usually it would run a bit rough after warmup because that's when the o2 really kicks in when hot and sends the info to your computer for it to tell your electronic fuel components what to do as far as the correct mixture,etc.
Mine was surging, and getting some crappy gas mileage as well as a rough idle once the car got up to operating temp.I could also smell a sulfur-like odor sometimes when at idle and noticed that my o2 sensor light went off a couple of times and although it didn't stay on, I figured it was either GOING to go or is gone. I replaced it and she's running fine cold or hot. How's the ride when you're on the accelerator (on the highway)? Does it also surge or use more gas than usual? Sometimes it may be sluggish when you hit the accelerator although the tach will show you using a crap load of gas to get going. Hope this helps.
also try a search on rough idle or similar and see if that will give you some ideas. Good luck!