Vease,

I would rule out your battery issue since you are on it....but this almost sound like a dirt issue. I would check the in-line fuel filters for crap as it sounds like an obstruction that effects the higher fuel flow demand.

My 50cc behaved exactly the same, until I found what looked like the white part of a dandeline or milkweed seed on the screen in the filter. Perhaps something on the diaphram too?.

Please keep me posted....

Tom