05 250xs stutters at top speed

I was wondering if anyone could point me in the right direction. My 2005 250xs planes fine and runs at medium RPM fine but as soon as I get on it it seems that it will run so-so for a short distance but around a 1/4 mile or so it starts sputtering a bit like it is running out of gas. If I come off the throttle to about 3500-4500 it levels back out and runs fine. As soon as I get on it again it does the same thing and hestitates and surges hestitates and surges and so on. Any suggestions? Thanks

Have a buddy run along with you.... when it's acting up, have him pump the primer bulb to see if it "attempts to recover".

If so.... it's a fuel delivery issue, such as:

-Fuel line/Primer Bulb/ Anti-Siphon Valve
-Pulse Fuel Pump
-Restricted Fuel Filter
-Low Pressure Fuel Pump
-Debris in VST

It would be nice to know at what rpm {exactly} it's happening at....if WOT, you might be on the limiter.....