Try upgrading to schedule 80 fittings.

gowings, I am unclear about the system components but here goes. If it is a poly pipe line that is splitting right after the fitting goes into it, then either the fitting itself is too large or you might be forcing it too far into the pipe. Try to push it past just enough barbs to make a snug fit and secure it with a hose clamp only enough to prevent leaks. This next might be a little far-fetched but let's say that this particular valve is so aligned on the manifold that it forms a straight line with the supply line. I am thinking that when this valve alone is opened, there might be an extra pressure which could cause the line to split after a while. Clearly, I have run out of ideas.

Ronalawn's comments are good. Also, check the barb fitting to see if there is any stray plastic from the manufacturing process (or other damage) that is possibly scoring the pipe when you push it on. Any scoring action could weaken the pipe and cause it to later split. Kind of like scoring glass or tile makes it split easy.

Thanks for your assistance. I've once again changed the fittings and checked the fittings used with a fine tooth comb.
Here is a link for a picture I took

Here is a link that might be useful: