'98 F150 rough idel problem

My 1998 F150 seems to idle fine in park. But when I put it in gear and come to a stop light it starts to idle rough like it wants to stall. If I put it in Neutral or give it a little gas to increase the rpm's it seems to idle fine. Where can my problem be?

Thanks,
gary

I would suggest cleaning or replacing your idle air control valve (IAC). It is attachec to the throttle body.