3 cyl auto no reverse or 2nd when cold good when warm

I have a 1990 Pontiac firefly with a 3 cyl automatic. It will not go into reverse and will shift 1st to 3rd when the car is cold. once it is warm it shifts fine. it is puzzling. When it is warm shifts are crisp solid. I am thinking it is not worn disks or band. I am open to ideas as I would like to get this fixed before winter sets in
Thanks

I have a little more info now. If I shift into low it will shift into second, So I now have all three gears but still no reverse until it warms up. I don't seem to be getting any takers, and this is so odd. I could use some enlightening please.

I had about the same symtoms and solved them with a can of seafoam-my valve body got clogged up when I changed ATF and some gunk got loosened up-the only other thing I can think of is that computer thing that shifts your gears-