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