This is a 1991 420SEL just turned 90,000 miles. Last week I noticed a slight skip in the engine which has gotten worse to the point that it felt like it was running on 6 cyl. I changed the Distributor cap thinking it might help but made no difference. Now it wont even fire at all. Any suggestions? I've heard that once they reach 90 to 100 thousand miles that you should change the timing chain, is this usually the case?