Own a 1991 300E 2.6L at 128,000 and have been having problems with shuttering and stalling at idle. I have gotten the coil, plugs and wiring replaced along with fuel injector treatment from the tech guys with no success. The guys at the shop think it is a coolant leak in cylinder 6 through the head gasket but the more I read the more I think it may be something fuel related. What do you guys think I am going to check to OVP to see if the fuse has blown and will get back but wanted to see if I could get feedback beforehand. Thanks!!

One note. At idle I'm at 500 rpm or below and have always thought this is a little low. Is the norm 750?? Should I increase the throttle to try and solve the problem??