I have a 2001 Chevy monte carlo ss. I have never had problems with my transmission until now. When I first start it up it shifts great and accelerates great. After about 30 minutes of driving it starts to shift hard in first and second gear. It is when I come to a complete stop and then start to accelerate it shifts hard when first starting off. This only happens after about thirty minutes of driving and then if I shut off my car for a couple hours it shifts fine again for the first thirty minutes of driving. Can someone help me what the problem is???