Shifting Problem

I am having trouble with the shifting on my 86 elky. I think its a 200r tranny? It likes to shift directly from 1st to 3rd, only when its good and warm and not getting on it. If i get on it up to highway speed, it shifts fine. But if i just let it do its thing from a stop, it skip shifts?? This problem comes and goes but on an average of once a day. I already installed the lube additive. Any Ideas???