What codes are set in the computer? If any? Something is so out of wack that something must be showing up in the system self diagnostic. If possible post them here.

Buy one of those cheap screwdriver looking spark indicators and the next time it does not start for you, check for spark before you have it towed.

Wild thought process to follow.
Car does not start.
Towed to mechanic.
Car starts for mechanic.
Car ALWAYS? starts for mechanic.
Get car back.
Car begins intermittantly not starting.
Car finally does not start.
Car towed to mechanic.
See above.

The key is what is different between the mechanic's and your starting process and how the car is driven not driven between starting attempts.

Items to look at.

How is car towed? Front wheels up? On a flat bed? To get serviced the angle of the car is changed dramatically at one point. This puts the charcoal cannister into a different orientaion which clears something or begins the purge of the cannister. Flooding can occur with a malfunctioning charcoal cannister. Which is sucked out and floods the car as you try to start it, then all gas evaporates and the plugs dry by the time it gets to the mechanic.

How is the car parked. Is your driveway sloped? Fuel level. Fuel pre pump or faulty check valve.

How many keys on your key ring? You have a thousand, mechanic only one? Do you give the mechanic the valet key? To have the car towed, one key is given and used to lock/unlock the steering wheel which resets the switch?

Like I said, wild thought process.

Duane

PS. This is one when the cause is determined, you HAD better post it here.