When driving over sixty mph there is a lack of power and it misfires

I just bought this van. Gave her a good flush with flush cleaner, coolant and distilled water because it looked like it hadn’t been done at all. Bought a new thermostat. Oil has been changed, air filter done, I have put seafoam in the gas. I will be adding top engine seafoam to her as well. I’m currently putting new o2 sensors and fuel filter on her. When it gets hot the coolant stays at the fill line. My other cars it went down when vehicle got hot and went back to normal range when cold. What’s up with that? When driving, mostly over sixty or up hills, there’s a lack of power. It misfires. No codes being pulled up at all. She’s not overheating according to the temp gauge. There’s a humming noise coming from the engine area when she’s in idle. I think that sums it up. Help.