A dirty flame trap and crankcase ventilation system won't cause this problem, but it CAN cause other problems. It's worthwhile to go through it, end-to-end, one day.

I remember working on a 740 that had a similar acceleration problem. The problem had gotten worse and worse to the point where it was dangerous to drive it up the expressway on-ramp. After 50 mph, it was fine -- but couldn't get get outta its own way from a dead stop.

The shop tried all the things you mentioned.

Long story short ---- the alignment pin between the cam and sprocket had sheared, the the cam was slowly changing timing (probably retarding) so the performance was degrading, just as you described, a tiny bit each day.

I have no idea if this is the cause of your problem, and I would NOT dive into the timing belt just yet. But file this idea away for future reference if nothing else helps.
--
Don Foster (near Cape Cod, MA)