Q: My van if I'm lucky goes 20 mph for a few minutes then it crawls.

It first started after an alternator replacement. I'd be driving and after everything gets hot I'd loose pressure in being able to drive like a vehicle does when it gets hot but yet it wasn't overheating. I did an oil change, radiator flush, thermostat change, and at one time put gasket sealing stuff when it started doing it more often. It seems when things would cool off it'd be fine again until it'd get hot. Thought maybe fuel filter but not that. Then noticed the transducer had melted so replaced it. One person said purge valve canister but not that. Another said idle housing but looked it up and there's no such thing but different components to the idle. Now my battery has been draining within a couple hours. Even to a battery that was only three months old and now a even heavier duty battery. When I drive now it goes good in a parking lot but hit the road and i cannot get up to speed. At most sometimes 20 mph but soon to barely crawling other times crawling. Can you help please?

My car has 245000 miles.
My car has an automatic transmission.