t400 tranny sticking between gears @ low rpm?

[WinDancer]

I have a 396 in my truck. Under feathered throttle conditions my tranny reaches a point where it constantly [like 40-50 times per minute ] keeps changing from 1st to 2nd and then back again, over and over and over.
With moderate throttle it works perfectly.
Had a new B&M adjustable modulator put in with no change- the mechanic says the 400 has no carb-to-tranny cable to adjust it correctly like the t350's do.
Has anyone actually had this problem? Were you able to get it corrected? What did you do?
Thanks,
Dave

[mtdave2]

still sounds like a modulator to me.. id crank the sensitivity down a bunch and see if it stops. What tells the trany to shift is the modulator.

[chevy_mike]

The TH400 does have a vacuum line that hooks to the modulator. Make sure there is not a leak in that line.

I am assuming you don't have some wild cam installed in the 396 that is lowering the vacuum signal too low. You should be seeing 14"+ at idle.