1999 LHS idles hard at startup

My 1999 LHS will from time to time idle rough at startup. After several seconds of patting the gas pedal it will smooth out. After this the car runs as smooth as a new car. 3 or 4 times now the car has not started. I have it towed to my mechanic and it starts perfectly every time. The last time it was in the shop they drove it for a week and it never acted up. Last night the car did not start. While I was waiting on the wrecker I ate dinner then tried it one more time before the wrecker arrived. It started perfectly. I hae had the crank sensor replaced but that did not solve the prolem. I welcome any thought anyone might have.

thanks

Is there a check engine light on? The 2 issues you describe may or may not be related. Does the starter turn over? Is this a 3.5 V6? Has the car had any tune up maintenance?:popcorn:

The check engine light as never come on. The starter always works normally. I have not had maintenance on the car ever as far as spark plugs go. (Im obviously not a mechanic) I do the regular things like oil and filter change and air filter change.
thank you for your help

Forgot, yes this is a 3.5 V6