loss of power under a oad is a good indication of a clogged catalytic converter
I would say it's a slipping clutch
Losing power is a lot of times due to a clogged catalytic converter, have that checked out.
Yes, if it just started in time you will lose power and speed, replace it, it is breaking up inside.
Sounds like you belt came off.
The transmission doesn't lose power, but it quits getting power if the clutch starts slipping. Without going into detail about the physics of a clutch, the most energy is being transferred through the clutch when in the highest gear thus it is fifth gear where slipping is first noticed. My guess is its time for a new clutch.
Fuel Filter? Air Filter?
Yes, if you had to make a sudden move at highway speeds you may lose control.
that would probably be the transmission. It may not qite be working right.
Not downshifting properly Plugged catalytic converter Needs tune-up (new plugs)
what would cause a 150cc scooter to lose power?
The obvious answer would be that there would be a leak somewhere...
Is it doing anything? Lights? check the ground line? Bad Unit? Warranty?