Why does a 1999 Ford explorer seem lose power when it gets hot

It shouldn't " get hot " . It should reach an operating temp. If you're saying it does that once it warms up then I would have the codes read even if the light ain't on . Ignition upgrade ?

I would check the spark plugs and wires to start off. Look for any damage whatsoever. My truck had a misfire because one of the wires had just been worn through to the metal. Went to the parts store and picked up new wires and plugs for 40 bucks. Just pray it’s nothing more. It sounds like it’s a misfire which isn’t typically a big deal. Best of luck to you!