RMVB 727 issue

Is it a reverse band apply, I.E. compression braking valve body? Try shifting into neutral at a stop and then back into first to see what it does.
HI Garrett, thanks for your reply. It has no band apply in first, but I will try as you mentioned tomorrow to see if it makes any difference. I should also mention that I did not have this problem when new, it developed after about the first 20 or 30 miles.