Couple of the usual questions: What tank set up and what prop?  I'd suggest bench running the engine on about a 9 x 4 prop. Run it a couple of minutes in a rich two cycle and let it cool down, then do the same again several times.  Idea is to run it fast, but well lubed.  If it gets hot and starts to slow down, pull the fuel line off.  Hopefully you are not using a Fox NVA.  I run St NVA's and there are other good ones.  Fox should run fine on muffler pressure and uniflow tank. 

        Darren, my experience with Fox 35's is that they just don't cut out. In the event that it does there's usually a reason. Now, the engine being new brings a whole lot of issues in itself. In the event your using the stock plug I would get rid of it immediately. I've seen more issues with those plugs than any one particular problem. I've even had those plugs go fresh out of the package. I still use them in a pinch but this way I do know what I'm in for. They can cause this erratic running and shut off. A good hot plug is in order. I've had backplate gaskets leak which can do the same as you stated. The one difference here is that you've stated hemi so I'm guessing you have the stuffer plate as well. These backplate are far better quality then the stock and have more area for the gasket to lay on without getting squished out. This doesn't mean that the case surface isn't true. You would be surprised at how irregular the case surface is if you leveled it on a flat piece of glass with sandpaper and oil. This practice should be a prerequisite on any Fox .35 new or not. Based on the other info you've mentioned I would also try different fuel as an experiment to rule that out as well. I've seen fuel work in one engine and not in others. Engines would start right up then just quit like you've mentioned. I would just try it due to the simplicity of the experiment. Vibrations can cause excessive foaming of fuel and the engine just quits. This can be the most frustrating. If your using the older style needle valve assembly make sure you put a small piece of tubing on the threads butted up to the spraybar so no air is leaking. Ken

well after checking things and fitting new nva making sure no air leaks anywhere changing fuel and also changing plug still no change and that after another 45 min run time. will hold a 2 cycle run but as soon as you richen to try get a 4-2 break it'll stop. wonder if it is cooling plug or not. when it stops put glo stick back on and starts no problems at all. no tightness in motor so this has got me puzzled

I had just about the same problem that you have described just yesterday with a Fox that had some time on it and always been a pretty fair runner. To make a long story short it was the back plate gasket. I wish Fox would go to an o-ring and loose the cork gasket.

Darren; What plug are you running? Sure sounds like a plug problem to me, since it quits when you richen it up. My Foxes and McCoy's will both do that with a cold plug. If you are running a hot plug, you might try another one just in case you got a bad one.