Truck Forums - 98 Sierra starting problems

I will try to keep this short, my 98 Sierra has trouble starting. One morning I went to start it up and it sounded terrible when the motor would turn over, back firing thru the throttle body and it would turn over then stop. Try again and the same thing. Took to mechanic and it sat for 24 hrs and he cranked it without any problem. He checked it over and could not find anything wrong. 3 weeks later the same thing and 2 weeks later the same thing again. mechanic still can not find anything wrong, after it sits for 24 hrs it will crank. One thing strange, it always happens after it has rained that night before.
A friend said here is a sensor on the harmonc balancer that has something to do with the timing that may be bad or sensitive to the weather. Is this correct and anyone had the same problem, I can't afford this much longer. Any suggestion????