90 chev shift when at red light

my 90 chev 1/2 ton has had this intermittent problem for some time now.I will come up to a red light and while waiting for the light to turn green,the vehicle will seem like it makes a shift and clunks into another gear.The truck actually lurches ahead a bit.When this shift happens,the idle almost always increases as well.any help would be great.thanks in advance.I have heard this may or may not be the torque converter??????

It kinda sounds like the problem may just be the splines in the slip yoke. It could be mistaken for a shift of gears.

I had a 95 suburban that the slip yoke on the driveshaft would bind on a rapid stop from the axle twist causing the driveshaft to pull slightly away from the transmission shaft. When you released the brake to proceed the pressure was released from the splines and I could hear a clunk from the driveshaft returning to the normal position.

I removed the driveshaft and put a small amount of grease on the splines and solved the problem. There are other more permanent solutions but I have never done them.

James

thanks for your reply but this is definately a shift.The truck litteraly jumps ahead somewhat.I doubt that a loose spline would cause the truck to jump ahead.Again thanks for your help anyway.I willl be replacing the u-joints soon so ill add the grease to the spline and see if it helps.