Tranny

I have a 1994 chevy pick-up with a 700 R-4 The trans. likes to slip into passing gear at almost any speed. It does this erradicately, also I live in Minnesota, it is below freezing temps here now and I didn't have this problem last summer when it was warm. I had the local Gemini repair shop do a complete flush and fill of the tranny. It didn't help any. What can I do?