[westcott cobra]

With a vacuum meter and a wideband A/F Controller i tried to find the root couse for the same problem as you describe it now.

With my FE it was definitely a mismatch in carb tuning because everytime I felt the bog I was always rolling in idle rpm and just pushing the throttle slowly a bit to keep cruise speed. Vacuum went down imediately in these situations.
Corresponding to the suddenly low vac the A/F meter showed a very lean condition about 16:1 or higher.

The bog went after finding the correct powervalve but was still there just a bit. I adjusted idle to 800 and with that rpm the problem was solved completely( before idle was at 650) and the A/F meter showed never leaner as 14:5 to 13.8 at low and middle rpms and about 12.3 to 12.5 at WOT.

Engine had aluminum flywheel and the car has very long rear end with 2.88 and 5 speed tremec.

200-300 rpms higher in high gear rolling around and nobody could have noticed the lean condition.

First step in tuning was changing the primary idle air bleeds from .033 to .036 because the car idled too rich and the adjustment the four corner screws allowed was to small.

Westcott