Since you're asking for any help... I have an 87 420 also with 107K. Had symptoms you describe that started without warning or reason. Felt like it was missing a beat at idle (very rough for about a second every five seconds, oil pressure dip, RPM dip) and sputtered on takeoff like an old Peugeot. Threw a bottle of Techron in the tank and took it out on the highway and did some hard acceleration. Also put it in park and reved it into the 4-5K range for a bit. Took care of the problem straighaway. Not sure if I had a fuel line/filter/injector prob or if just running it out took care of it. Not too scientific but worth a shot?

John