Q: Driving problem

So I have maintained the truck with all its services like tune ups, oil changes, fluid changes etc. So recently I tried to pull something heavy with the truck but it could pull it, but I did give it a bit of tries before I gave up and stopped. The truck now has some problem while driving. It shifts very well through all four gears but once it gets to 4th gear which is like around 40 mph, the rpms drop and don't move from 1500. I have to press the accelerator almost half way for it to start accelerating but when I do that the truck kicks like if I ran over a small pothole and that's when it accelerates. I'm not getting any check engine codes or anything. Also I had a heater core hose bust and it covered the whole top of the engine and it got into one of my spark plugs but I changed them all out including all of my ignition coils because the coolant that had spilled ruined one of them so I decided to replace them all. I just wanted to know how to fix the kicking problem. Thanks

My car has 320000 miles.
My car has an automatic transmission.