I have a whirlpool dryer that will start sometimes and other times it won't I have already had apart and cleaned all lint and made sure the moter is fine

Are there any other symptoms?

When you say it will not start is it totally dead or does the motor buzz (tries to start)?

Is there anything you do to get it started or is it just random?

Sounds like the motor is overheating and tripping it's internal thermal protect. You then have to wait until the motor cools down and the protect resets before you can restart it.

Try turning the drum in the correct direction to see how easily it turns.
You may want to try this on a couple other units to get a feel for it.
I would do this both when the unit is cold and after it warms up.
Could be something is putting too much drag on the unit.

Clean the motor it may be clogged full of lint.

If there is a seal between the blower and the motor check that it is in good shape. Could be that hot air is blowing back onto the motor.
Check/clean the vent system.

If all OK then odds are that your motor is on it's last legs.