99 suburban 5.7 4l60e k1500 4x4. when driving if i just ease off the line at lights. it seems to shift ok not great but ok. but any time I accelerate harder or wot 1st and 2nd sometimes even 3rd will wind out very high rpms before shifting. sometimes it won't shift at all until i let off the accelerator to get it to shift. (recently replaced shifter solenoids after they went out at that time i added filter fluid Corvette servo with Sonnax upper servo and a external cooler none of these are directly related to the issue at hand just a f.y.i. on recent additions. this has been a ongoing issue for awhile. any ideas would be greatly appreciated.

 

A friend of mine suggested a stiff fan clutch could this.