When driving over thirty minutes speedometer acts erratic, cruise control stops working and transmission starts down shifting

Okay, so when I start my truck and drive it there’s nothing wrong with it. Until I drive it for more than 30 minutes. Then the speedometer starts jumping. I’ll be going 55 and it jumps from 52 to 60 and it cuts my cruise control off. And the longer I drive it it gets worse to where it goes from 20 to 80 rapidly. And then my transmission will start downshifting. So I went and had my codes read and cleared. But after they got cleared my truck wouldn’t come out of first gear for awhile. And this is where it gets crazy. I go to park it for the night and then drive it in the morning and it runs just fine with no problems at all. So I really need help.