My mom has a 1994 S420..has 200k.... It has developed a chug and now has hesitation. When you first start it up there are no problems and is smooth, once under load the problems begin. If you change out the spark plugs the problem goes away for a short time. There is a severe lack of power off line but does alright once it gets up to interstate speed. The local dealeship told her to run 85 octane gas to fix the problem. Also the check engine light is on, according to the dealer that is not repairable and will always be on now...... We have had several problems with the dealer here, but the smaller shop said my moms car was out of their league..... I need help here.... It acts like a timing issue, or a low compression issue... Am I off here? My thoughts are head work/piston rings... maybe timing oe ECU... The car is currently at the shop and we will get it back tomorrow, I will do compressin checks, fuel pressure checks etc... any suggestions? or has anyone had this same problem? Thanks in advance, Aaron