My 98 420 has run perfectly until today and I'd appreciate any input on what may be the issue. Upon accelerating onto the interstate it hesitated and stumbled a bit but it seemed to straighten itself out for the 15 mile trip and no problems while on the highway. After exiting the freeway and accelerating up to about 45 it again stumbled and was very sluggish, had to give it a lot of pedal to keep going. When I stopped it nearly shut off but caught and rev'd up to about 1500 and then settled down to a normal idle. When I started out it didn't shift like it has before, stayed in first gear until about 2500rpm and then shifted but not at all smooth as it has always been. Thinking this may be an issue with bad gas I put 2 bottles of HEET in and let it sit for about an hour. Drove it again and the problem occured again but there seems to be NO regularity with it and when I step into it it runs like a scared rabbit through the gears. It seems to hit after I go over some bumps that slosh the tank around. 3/4 tank of fuel. Always use Premium of course. Has me baffled, any ideas? Car is in excellent condition, all service intervals met, I baby the car and keep everything up to snuff, etc. Has 166000 miles but normally runs like new. Appreciate any input and thanks in advance.