1999 ML320 goes dead; next morning starts OK

1999 ML 320: On March 1st I backed out of garage and started forward and it died - tried to restart - no starter noise - nothing. Hauled to mechanic who put it on a lift and it came back to life (took about
5 hours from time it happened till I got it to mechanic). On March
18th I stopped at a store, came back out, same thing - would not start & no starter noise - left overnight - it started. Took it to dealer, they replaced crankshaft sensor (tested bad). Yesterday evening I went to start car - same thing again. This morning it started OK again. Drove it to dealer this afternoon. Dealer said they have no idea what it is - can?t do a computer check on it unless it is displaying the problem! I have heard that possibly my key may be going bad (?) - why would it be so intermittent? One of the other topics here mentioned that 2 of 3 bad coil packs could cause a non-start situation. Don?t know what those are. Thought it might be heat related but when it was in garage overnight and would not start - rules that out. Any ideas on what to check next??