Engine/Transmission (1998.5 - 2002) wont go in overdrive sometimes

[Lair]

Hi, new member and this is my first post although I have been reading for quite a while. Anyway, Three times in the last 2-3 months I had problems with overdrive going in and out. The first 2 times it did it around 50 mph but if I went faster it would be fine. Yeserday on my way to work it wouldn't go in to overdrive at all, it would try but kept going back out. But at lunch and coming home it was fine. Also durring this 2-3 months it seems like at times I have to really give it fuel to get going, Taking about 10-15 seconds just to get to 30-35 mph. My truck is a stock 98. 5 with 116,500 miles. Haven't done any hauling with it. Thanks for any help.