I had an express that acted similarly. It would run up to speed (27-28mph) but would then power would "go in and out" at about 20 mph. Although it had the stock airbox and a new filter, unbeknownst to me, it was missing that plastic, framework-like piece of a grid which held the filter element in position. Without that piece, once the engine came up to speed, the vacuum draw into the carb was enough to suck the foam up against the intake side of the carb. This then restricted the volume of air going into the carb. Back off the throttle, the draw lessened and the foam released from the carb at which time the motor would run fine until full throttle was applied and the power would then "go in and out". Odd, rare problem but worth checking.