Harsh shift in reverse only when hot

I have a '90 LeSabre and the car has an extremely harsh shift into reverse but only when up to operating temp. or hot. When cool or when first backing up it shifts nice and soft. I was wondering if this is probably internal or could a failing vacuum modulator cause a similar problem?

My hose cracked and particially fell off the vacuum modulator a few years ago and I had a hard downshift from 2-1. Other than that no problems. Any ideas would be appreciated.

Reverse is a seperate function not affected by the modulator, this is internal, at times a worn axle or trans/engine mounts can cause this, but that would not be temp sensitive.