1999 Chevy Suburban hard shifting

After a certain time of driving the truck starts shifting from 1st to 2nd gear very hard, a hard jerk and knock. The shift from 2nd to 3rd is not as bad but it is a harder than normal shift.

There is no consistancy in when this happens. On one day I can drive fourty-five minutes and it starts doing it. In the same week I can drive the exact same route and go four hours and then it starts. This is not heavy stop and go but some. I swap up driving so I dont use it all of the time. Just last week it started doing it after 30 minutes, I drove for about 45 minutes more then stopped to get gas, about 10 minutes at the most, then drove for two hours without it doing it any more. It seems the hotter it is outside the quicker it starts doing it, but the engine temp is never any hotter then normal when it starts. Sometimes I can drive three hours and it never does it. As I said above no consistancy on when it starts. All other times the tranmission shifts fine.