Carb Malfunction??? Help

[RocketChevy56]

I finished my new 355 and got her running about 4 weeks ago. I used the same carb as my last engine (650cfm street demon) I took special care in storing it in a carb box and put it up and away from anything that may harm it. After the first 50 miles or so of running it on the new engine, I've lost the ability to cruise above around 55mph. If I do, it bogs down and cuts out. Sometimes I can cruise just fine at 65+ and then out of nowhere it just give up. If I shift into neutral it acts normal and smoothes out. so it happens under load only. Could my floats be sticking? Perhaps a blown power valve diaphram? Might should have stuck with my reliable Holley.......