2000 Sierra Auto Tranny Bang Shifts 2nd And 3rd Sometimes???

I am thinking its a sensor but it's not my truck so can someone tell me what would cause this. Its a 305 with 4 spd auto with around 100 K miles on it in good shape and no big history of towing and she has owned it since new. Once in a while when warm it slam shifts very hard 2nd, and sometimes but much less 3rd. Usually if you shut it off and restart it goes away as mysteriously as it appears. Of course it comes and goes . The oil has been changed at 50K and is due for its next one. The truck has done it for quite a while now. Is there a sensor that causes this and if so which one. At present I don't have access to a code reader.:banghead::banghead: Thanks in advance.

This ca n be caused by a bad press. control solenoid, a worn valve body at the tcc valve bore, a skewed tps also, a full scan with a trans function scanner should show something.

Thanks Maxwedge, my tranny experience never got beyond the vacuum downshifts and worn bands. I hope the guy she goes to has a tranny scanner. He is a good old boy who has a small shop and does about anything but those diagnostic goodies have a hefty price tag so you know how that goes. Gotta draw the line somewhere. I will pass it on to her and see where it goes.