I have done a lot of forum searching and reading up on this issue - number one cause of stuttering/bucking seems to be resonance flaps, but indy mechanic says they're functioning OK. I've not used him before (came recommended, does enough MB work to have a StarTek system, and I've seen plenty of W123s and W124s at his shop) so I have to take that observation at face value. Also no codes thrown, as malfunctioning flaps tend to do on a '97, but some guys with flap issues have reported that codes were not thrown for them either.
Regarding K40 - from everything I've read when they start to get flaky, they cause stalling at highway speeds, failure to start randomly, and some other weird issues until they die completely and the car is stranded. Gasser MB owners with bad K40s have reported poor throttle response though, so it's not out of the question. I've been looking at the voltage regulator, fuel filters etc first. BUT now that they've been eliminated I'm willing to roll the dice on the K40. Besides, from all the K40 posts I've read, they're handy to have in the glove box.
I did a search on Pelican Parts for K40 relay, but don't see it. I suspect it's not officially called that - is this it ?
1997 Mercedes-Benz E300 Sedan - Switches, Motors, Relays, Fuses & Wiring - Page 3 The "relay module" P/N 210-540-03-72-M56
Thanks to all to responded - this is driving me crazy!