straight shift truck loses power when starting from stop at traffic light. on 1999 Chevrolet S10

The engine runs fine and I can get the truck moving by down shifting but it won't go over 15mph. Once the engine cools down it will run fine again. This is an intermittent fault

Asked by for the 1999 Chevrolet S10
Any CEL on? Have you had the fuel pressure and volume checked? May have internal trans probs related to hot fluid?
There are no warning lights of any kind. The gears still shift smoothly and I can get a little bit of power every time I downshift so I have always been able to get it off the road.
I would have the fuel pressure and volume checked and also have it checked for clogged exhaust causing too much back pressure.