Toyota 1994 4X4 160,000 miles

I have a 1994 toyota forerunner with 3.0 liter v-6. It has 160,000 miles, however I had a rebuitl grohms engine put in aruond 155,000. I now drive approx. 50 miles oneway to work daily and have noticed it has a hard time staying in gear. I will be driving down the road at 70mph with rpm just below 3 then all of a sudden it will change gears, bog down a little but manage to keep the same speed, but the rpm shoot up to 4. It may do this going up hill or on a straight stretch. I also noticed it is worse on cold days. Any suggestions?