what could cause studdering/chugging?

[Shane@DBPerformance]

Usually with higher than stock boost, you run a tighter gap. Could be plugs wires also. Yea, often a misfire shows up as knock, but not always. Have you guys increased the timing at all with the E85 yet? Too low of timing can cause misfires also. Adding ethanol or methanol to your setup will cool your combustion chamber temps a lot and it might take more timing to just get it to fire cleanly, but you don't want to go too crazy.