Friend of mine has this on a Stratos and when he gets to WOT it will fall from around 5500 rpms down to 4500 or lower without warning. He can get it back to wot by slowly increasing the throttle a little at a time. Low pressure pumps are not leaking and primer bulb is good. He recently had it in the shop and they claim to have cleaned the VST filter along with the OX2 sensor. ALso put a new starting battery on none of which resolved the issue. Any ideas ? thks