Randomly fails to start after being driven for varying lengths of time.

I drive an automatic SL2, am the original owner, and have kept up with all required and recommended maintenance. Over the past 9 months, I've been having problems with the car not starting. It starts just fine when it sits overnight or for a period of time. For example, I'll drive it to the commuter lot in the morning with no problems. When I get back to it in the evening, it starts right up, I drive home, and park it. If we head out after that, sometimes it doesn't start at all. When I turn the key, there's no sound or action whatsoever. Trying to jump start it doesn't do anything. All the lights and electrical work in the car just fine. It simply doesn't start. This doesn't happen all the time though. It will go days to weeks between occurrences. If I let it sit anywhere from 15 minutes to several hours (or overnight) it starts up just fine.

The same thing happens if we are driving around town running errands. The car starts just fine when we leave the house and even when we start it up at a few stops, and then suddenly it doesn't stop. It doesn't matter how long we've been driving it or letting it sit.

No for the challenging part. I can't replicate it, and the repair shop always says they can't find a thing wrong with it because it starts right up for them when we've had it towed to the shop. They've kept it overnight, driven it around, and every time it starts up just fine for them. They've test the battery, starter, cables, and who knows what else and haven't been able to find the problem. To make matters worse, we've never been able to get it to a mechanic when the problem is happening because it's always happened after mechanics hours or on the weekend and when a mechanic does get to take a look at it, the car starts right up.

The problem is happening more and more frequently with the gap between occurrences getting shorter.

Any suggestions or ideas are greatly appreciated.

Thanks,
John