After conversion, Paramount OS is super squirrely
If the headset is good then you need look at tire balance  

The tire itself could be out of balance and also it could not be seated evenly.  

Examine how tires are seated.

Put it on a stand or upside down and see if the wheels hold in position. Rotate 1/8 turn and check all 8 positions.  

For the rear you can spin the pedals - at high speed does it wobble?

If not balance then pull the tire and remount it.