Hi JP, thanks for your input. My gear changes are a tad rough at times, and it has been so since I bought the car 2 years ago. I would say the occasional harsh shifting is not yet to the extent to requiring a tranny overhaul. I've already replaced my ATF, fliter, and vacuum modulator in the past year.
I like what you'd suggested - "a softened/cracked hose that leaks briefly, then shuts as it is distorted by vacuum" - it is similar to Steve's suggestion that it might likely be a variable vacuum leak. Fuel delivery seems alriight, the pump is buzzing, the fuel filter is new.......I agree its more likely a vacuum problem.
In fact, I just got back from the mechanic's shop for my regular servicing, and my mech also thinks its a air valve problem rather than fuel related. But because I'm running late, so I shall come back another day to let him have a thorough check on the vacuum lines and valves.
Steve, I've located the black decel dump valve, but how do I check if its leaking or not?
1979 W123 (200/M) Midnight Blue
1983 W126 (280SEL) Dupont Deep Black (Previously Pristine Silver)