Just for reference... the Spree has a double sheave (pulley) belt drive just like a snowmobile.
I think its fuel related.. mostly because he said the problem happens most when its cold, and gets better as it gets warmer.. which is about the opposite of what happens when a CDI is going bad.
Maybe the vacuum operated petcock isn't operating correctly from a leaky diaphram or hose?
If it goes 35mph it is all in pretty good mechanical shape.