Trans 3 gear upshift

99 grand prix gt. 4t65e trans. 170k miles. way above avg for life expectancy. 1-2 shift is ok. will not shift into 3rd. if you keep increasing vehicle speed to about 45 or so, it will shift into 3rd and than immediately into 4th. 45mph is about 4k rpm. i can see the tach drop to 3k rpm for 1 sec or so and than it drops to 2k approx. it will stay in OD and does not slip. if i stay at 50mph or so and punch the gas, i can make it shift into 3rd gear and it will stay in 3rd as long as i give it 2/3 gas. once i left off the gas it goes back to OD. any theories why it wont hit 3rd on the way up but will easily drop from 4th down to 3rd whenever i want? yay, i know, its time for a rebuild.

there are 2 shift solenoids. one is referred to as the (1-2,3-4) solenoid and the other is (2-3). that sounds like its possible this could be issue. the trans code i should get is like P750 or so. something in the 75X range. car is at kids house and they have my cheap code reader but dont have time to use it. why i can downshift from OD to 3 though seems odd. why or how is it that i can sort of step around the upshift from 2-3?

got more info. 1 and 2 solenoids are used in different combos. if you unplug the main harness both solenoids are off and trans will stay in 3rd. if i can drive at 40mph or so with no apparent slippage, than clutchs/bands are ok and it may be a worn accumulator o-ring causing issues. which can be fixed by dropping pan. but the odds my trans needs a simple fix are slim.