I have a strange problem with starting my C240. The problem is very intermittent and has me pulling my hair out!! (What little there is left )

First off it has autostart, you know when you flick the key the starter continues until the engine runs.

The battery is maintained with a conditioner and the conditioning test puts the battery in the strong category.
It has had all the 12 spark plugs replaced about 10k miles ago.
It has a new Crankshaft Position Sensor and MAF meter.

It usually fires and starts immediately when the car is cold after leaving it overnight and when it is hot, but not always! 9 out of 10 times maybe!

After sitting for 7 hours at work it fails to fire, maybe 6 out of 10 times and when it does start it seems to "miss" for about 3 - 5 seconds then runs absolutely fine.

I would be delighted to find out what is wrong. I am tempted to take the car to a dealer to have a diagnostic run, but am a bit concerned that no faults will show up as the probelm is so intermittent... it does seem to be getting more and more frequent though.

Any advice much appreciated as this is driving me round the bend!!