2000 Chev S10 P1870

Hi all. New member. I have a 2000 Chev S10 2.2L w/4L60E 2wd. Problem is random shift to third at highway speeds. only between 60-65 mph. Other than that it shifts fine, no clunks, no slipping, or hard shifting, has reverse. Code is P1870, intermittent Check Engine light, Intermittent issue. When random shift occurs, still have trans grip when accelerating. Also when shift occurs, release throttle & shifts itself back to 4th, drives fine for a minute or so then downshifts again. Unfortunately, I don't have access to a scanner capable of reading trans data. Any ideas & suggestions would be greatly appreciated. Trans was rebuilt about 18 months ago, recent fluid/filter service, cleaned throttle body, fluid is clean & full, checked brake pedal switch operation & voltage, throttle cable is not getting stuck, TPS is clean & functioning, have yet to check shift solenoid & TCC solenoid voltages, Neutral Safety Switch, & Shift/ Gear Position Sensor is functional. Found a new Valve Body direct from Chev. for $365 & comes with new shift solenoids but I would like a little bit more info before throwing that money at it. Multiple trans shops cannot recreate issue. was there all day yesterday & even rode along with a couple techs & watched data. All seemed fine, but again issue would not pop up. Truck does have reverse. Thank you all for your time, & I appreciate any & all feedback

Hi all. New member. I have a 2000 Chev S10 2.2L w/4L60E 2wd. Problem is random shift to third at highway speeds. only between 60-65 mph. Other than that it shifts fine, no clunks, no slipping, or hard shifting, has reverse. Code is P1870, intermittent Check Engine light, Intermittent issue. When random shift occurs, still have trans grip when accelerating. Also when shift occurs, release throttle & shifts itself back to 4th, drives fine for a minute or so then downshifts again. Unfortunately, I don't have access to a scanner capable of reading trans data. Any ideas & suggestions would be greatly appreciated. Trans was rebuilt about 18 months ago, recent fluid/filter service, cleaned throttle body, fluid is clean & full, checked brake pedal switch operation & voltage, throttle cable is not getting stuck, TPS is clean & functioning, have yet to check shift solenoid & TCC solenoid voltages, Neutral Safety Switch, & Shift/ Gear Position Sensor is functional. Found a new Valve Body direct from Chev. for $365 & comes with new shift solenoids but I would like a little bit more info before throwing that money at it. Multiple trans shops cannot recreate issue. was there all day yesterday & even rode along with a couple techs & watched data. All seemed fine, but again issue would not pop up. Truck does have reverse. Thank you all for your time, & I appreciate any & all feedback
Unless you just want to throw parts at it.
You are going to need a good engne/transmisson capable scanner to find the problem.
If you do not have one find a good repair shop that does.
That code is transmission slippage that usually goes back to bad transmission.
Some people have band aided them with a valve body repair that helps for a while.