An intermiten problem at best

[Cope]

I thought I'd try the forum and see if anyone has any ideas, my shop is just scratching there head. Ill turn the key to start, all the dash lights light up but no action from the motor, absolutely nothing, no cranking noise or anything. I'll leave the car for a day or two and try again and it'll fire up. This problem only happens 3-4 times a year and runs as per normal. My car is a 99 but had a 996 3.4l crate motor installed 6 years ago, the problem seems to happen at the most inconvenient times and does it even when the motor is hot or cold. If anyone has any suggestions I would greatly appreciate it.......