Shift Problem

[daytonboi]

Yesterday my van did something weird. I got on the highway and attempted to merge and the van wouldn't go above 40. It seemed to be stuck in a gear not sure which. So i pulled over cut it off and sat for a minute. I tried getting up to speed so i could get to the exit and trying not to overrev the motor i took my foot out the accelerator and as the revs dropped it shifted. Long story short the van will shift fine anywhere just short of WOT. What could the problem be?
98 Astro AWD with 160,000mi.

Did some more driving today and it will shift through every gear at normal speeds all the way up to about 85% throttle and then after that it kicks down and won't shift back up again till i'm below about 85%.

This condition exist in all gears. if in first will rev till redline and then shift once i back off. second third and fourth the same thing. Only code thrown is P0420 which is the Cat i think.

[rlsllc]

How does the fluid look?

[daytonboi]

i guess alright...although it is probably original fluid...didn't smell burnt or anything

[LiftedAWDAstro]

Change the cat converter and you will be ok. A partially plugged cat will make these things act weird.