Hi Mennis,
High possibility the valve seats are loose.
The fix is to purchase a new cylinder upgrade kit.
I personally spent thousands of dollars trying to fix the original WC-1 cylinder. I even purchased special tools made to "peen" the seats into the cylinder. I had tapered seats made, screw in seats made, oversize seats made, purchased special bonding liquids from Loctite #640 & #620], etc, etc. I wasn't the only one spending money on trying to find a solution, there were several others that also failed to make it work.
The problem has to do with the grade of aluminum and the high operating tempature of the motor. The Aluminum expands much quicker than the valve seats, and once loose the seat loses its seal. Often the motor will run [after it cools down], however when the motor heats up it will bog and die. Many times I sat on my Whizzer waiting for it to cool down enough to ride it home.
BTW, Everyone one was sure they could fix it, I heard how they were the world's greatest or knew someone that was the best. I watched a lot of money and time spent on the problem, and never heard of a solution. However if you want to try I have a bunch of the WC-1 cylinders sitting on the shelf in my parts room.
I won't bore you with the details, but you might want to find the NE cylinder upgrade parts ASAP [they are becoming more difficult to locate].
Have fun,