My '01 silverado has recently had the trouble of not wanting to start. I will turn the key until it catches then release as normal. However, the engine fires for about a second or two and suddenly dies. It doesn't do this all the time. If I fiddle with it for awhile or sometimes let it sit for awhile it usually starts. Sometimes when I am driving the security light will come on and stay solid. My mechanic says he thinks it's a security relay going out or something that cuts the fuel. Any ideas?