1994 Suzuki Slips into neutral at stops.

Transmission problem
1994 Suzuki Two Wheel Drive Automatic 94753 miles
Garage name : cba garage

Hi Kenny.

Ok, I have one that is a bit weird. This car came in with a poor run Issue, we replaced a bad water pump and t-belt, a leaning Injector and a new distributor(Bad pick up coil) and wile the car runs fine, wile we were there we did a transmission flush.
The book calls for Dextron IIE, which is now Dextron III, all seemed to be fine.
The customer a month later is now back with a Intermittent problem of it coming out of gear at a stop (mainly cold).
I checked the car, and the fluid is full, I took it for a test drive through the west hills of Portland Oregon, at a stop on a 20* incline, it did as he said, it would not go into drive or reverse.
The Filter on this trans, according to Michell is a screen, we did not replace it, just used the flush machine, it is a bolt on type.
No leaks external.
This seems to be related to the cold weather, it is 35 and snowing here, and the customer said it started once the temperature dropped.

I am open to suggestions.

Jeff Miesen
Courtesy Auto.
Portland Oregon.