2003 bi-fuel (cng) starting issue
 

I just bought a 2003 Express CNG Bi-fuel van. All of a sudden turns over several times before it will start. Occasionally resets the clock to 12:00 when it does. Confusing part is that one time it would not stay on CNG but returned to unleaded (at least according to the gauge and fuel button). Is this just a starter issue or something else related to the CNG? Only once in last 10 starts did it start right up.