Car has 238000, still runs good. Every so often will not crank back up immediately, if you let set it may crank back up. Mechanics put on machine and shows not problem but can't depend on it to crank. What could issue be?

I would ensure the transmission is securely in "park" (or the clutch engaged on a manual transmission vehicle). I would check all battery cabling to ensure it is secure and clear of corrosion. Other than that, you have to catch it in the act and see what is happening via hands on testing.