1993 shifting into 3rd and 4th way too early?

[1500z71]

My 5.7 350 auto 1993 has all of the sudden began to shift into 3rd gear and sometimes even fourth gear around 15-20 mph (on average), causing the truck to lose rpms and power, therefore making it almost die on the spot. Seems to only do it when its colder most of the time. I can manually downshift to get the rpms back up but I know this disastrous in the long run for my transmission. What could be causing this? Is it a computer failure? I am going to a junkyard soon to try and snag a same model computer. It has my mechanic stumped.

 

[kkirwan71]

Do you have big tires on the truck?

sent from my glade air freshner

 

[1500z71]

Do you have big tires on the truck?

sent from my glade air freshner

275/70/18 stock 18s. Had them for almost a year