Not downshifting properly Plugged catalytic converter Needs tune-up (new plugs)
Gravity is trying to push the car down the hill. It takes more power to overcome that push of gravity. Unless you give the car more gas it will slow down.
I would check the point gap and timing first. If that does not correct it, I would check the carburetor float level.
Once you have achieved highway speed, you would stay in the highest gear, except for steep uphill or downhill grades.
Is the vehicles transmission downshifting to a lower gear as you go uphill? If not, that would be your first place to look. After that I would suspect that your engine timing is not autmatically advancing as it should be.
It could be your connection to your battery. If its bumped loose, you would lose all power, even while driving.
get an engine diagnostic scan.
A long bike ride that is partially uphill be would be considered vigorous physical activity.
the voltage regulator
Can be a few things. I would suspect a vacuum leak. Start by replacing all vacuum hoses. Replace one at a time, so as not to get confused. Very cheap repair. Also replace the spark plugs and wires unless they have been replaced within the last 50,000 miles.
vacuum leak
At what point does it lose power? As you're driving? When you come to a stop? plugged converter
A dodge pickup 1500 would stall out while driving because of a defective air filter.