To shift into OD, the OD band applies and at the same time the forward clutch
releases. If the clutch releases B-4 the band applies, the trans is in neutral
a short time. Under lower throttle, it might be lagging enuf and under not as
high a tv pressure so when it "catches" it clunks or bangs.
Another possibility, if the band applies B-4 the clutch releases, there's a temporary
hyraulic "park" situation where it tries to stop the input shaft from spinning till
the clutch releases.
I was thinking also that maybe the governor is sticking, but you're right it should
then affect all the shifts just like too high of a tv pressure. That throttle valve is also
known for sticking if it doesn't have the later stiffer spring, I think it's supposed to be blue.
So, the first thing to check probly IS that 3-4 shift valve. AODs are notorious for
sticking valves in the vb
. I've had that happen twice after a rebuild even when
was apart and very carefully cleaned and checked for free valve movement.
With the higher tv pressure and/or line pressure of higher rpms & full throttle, it maybe overcomes the valve sticking.
Do you know which valve that is? I've got a service manual showing a breakdown
of all the valves and parts if you need, I've got it scanned and can email.
If you're thinking of reinstalling that AOD at some point, here's some info that
may interest you as long as it's out.