the Great Gas Grill Mystery

A couple of weeks ago my gas grill quit working, or it might as well have.  The burners still lit, but with a flame so low it was basically useless as a cooking device.
I decided the problem had to be the regulator, and guess what, I still had one attached to our previous grill out in the barn.

I pulled it off and installed it on the new grill.  
Voila! 
Problem solved.  I was able to complete cooking our burgers.
It continued to work well until last night when it went into no-cook mode again.  The symptom was the same, but two regulators in two weeks just doesn't make sense, especially considering the regulator is a part that almost never fails. 
I decided to try some diagnostics this morning.  
The first thing I did was light the grill. All burners came on at full strength - I couldn't get it to do anything wrong!
I know something was wrong last night and it will probably happen again, but I have no idea what caused the problem.