trying to diagnose a transmission problem in a 2000 grand prix gtp with the 4t65e-hd (stock) transmission. the car shifts fine untile the 3rd to 4th transition then it osscillates up and down by about 500 rpms like it cant quite get into 4th gear. if i let off the gas a little then back on it shifts fine. happens randomly it seems too. ive checked all the fluids and they are okay at this point. ive heard getting the ecm re-flashed might fix or it may have a bad selonoid or somthing of an electrical nature. also heard it was some what of a common problem too. looking for a little insight. thanks