Weather sensitive tranmission problem?

I have a 1994 Plymouth Grand Voyager, in warmer and hot weather the transmission will sometimes not shift out of first or if it does shift normally, sometimes slam itself back into first. If I turn the van off, and let is set for a while, it will go back to shifting normally. This only happens during warmer and hot weather. I had no problems at all during the winter and cold months or very early in the morning before the temps get up to over 65*F. this does not happen all the time when it's warm But often enough.