An 86 740 GLE I think had the B230E carb engine.  If I am right what you are
describing is reminicent to a similar symptom that regularly pops up on the
340's with the crap Weber carb where the base plate bows slightly on the
carb where it bolts onto the inlet manifold (via a 1/2 inch thick gasket).
This gives a very slight air leak and causes just that which you have
described.

Check the carb to manifold bolts first but do not overtighten.  Next check
the inlet manifold bolts .    Does spraying WD40 at the carb/manifold
interface briefly drop the reves down?

Another thought;  I also recall that the 740GLE has an auto choke rather
than one one you have to physically pull out.  If it is a pull out type then
check to make sure the cable settings are returning when cable pushed back
in.  Check your throttle are is returning to its stop on the carb.  If an
auto choke could this be playing up?

Further thought - eratic idle - idle jet (presumably there is one) in carb
partially blocked?  Guesing somewhat but you never know.  You really need to
get to somebody who is a wiz on carbs.

Best of luck

Andy