4l60e slips in passing gear

Hi what I got is a 99 Chevy 1500 Express Conversion Van, What it's been doing for the pass 4 yrs when you floor it to kick it into passing gear it drops down to 2nd gear and slips until let up on the petal some then it up shift to 3rd & 4th as normal. I just don't floor it to pass anyone. It has only show a SES light code 1 time a couple years ago for the shift problem. I reset it to see if it would come back, still waiting. The other day I was crusing down I70 with the cruse on at 75mph when we went up a hill it downshift and slipped until I turn the cruse off, I reset the cruse on going down the hill and on the next one it did it again, on the way home it did not have the problem with the hills with the cruse on. The fluid level was OK and has deep red color. The 1st owner I talk to him 4 yrs ago, he said he had GM replace the Trans under warranty @ 35k, and it start having problems at 63k so he traded the van in. The 2nd owner said he change the fluid at 69k and did not have any more problems with it, It now has 109k on it or 74k on the trans. I was able to live with the do not floor it to pass someone, but now that I had the problem with cruse on I'm don't want to break down on the road someday to far from home. I would like to troubleshoot this before I put to much $$ into it. Thanks for any help you can gine me. Heff

First step is to get it on a transmission capable scanner and read out data and check for codes.
Post back any codes and shift adaptive data.
MT

Last weekend the route cause of my problems came to surface, the fuel pump quit working and after I replace it along with the fuel filter. on my test drive I kicked it down into passing gear and it acceloratered right up to 75mph shifted into the next gear just fine. I checked it 4 more times and it shifted just fine. This would explain why I was not getting any service engine codes and it was not getting enough fuel to run the RPM up high enough to shift to the next gear unless I let up on the petal, It now goes up to 4900 RPM and shifts, and yes it was over due for a fuel filter change maybe even from the last owner. Thank You anyway for your input.

You are welcome and thanks for posting back how it went and what you found.
4900 is a little higrpm for a normal shift.
On a normal take off what are the upshift mph speeds and rpm.
MT

Give or take 25--50 RPM 12MPH-1475RPM 25-1900 40-1900 50-1900 55-2100 Torq Conv lock-up cruse on 55mph 1535RPM Shifts out of Passing Gear 4700RPM 75MPH is this pretty close to normal? Thanks again for the feedback.

Normal upshift on normal take off.
18 mph 1-2 shift 28mph 2-3 shift 35-40 3-4 shift and lockup when at a steady speed in OD.
Passing gear up shift may vary some bit 4700 rpm upshift 3-4 is not out of range.

Make sure filter an fluid level when hot is good.
Only use dexron fluid.
No multi purpose fluid or other stuff.

Engine running and engine computer sensors and even fuel pressure have to be up to par for the transmission to work good.
Go over engine running real good.

To test transmission shifting and or slipping you need to get it on a good engine/transmission capable scanner and read out the shift adaptive data and slippage and the shift seconds delay.
Got of good test and info in a good scanner.
Good Luck and let us know how it goes.
MT