2006 Ford F-150 Engine RPMs to low

Hello,
My F150 has started doing something funny every now and then when I approach a stop. Just before the vehicle comes to a complete stop, the RPM's dip down close to 300 for a split second and then bounce back up to normal range once stopped. It hasn't stalled yet, but in stop and go traffic, it is anoying. Every so often, the engine trouble light comes on the dash - I have a an Edge Lightning Performance Module / Engine Code Reader installed on the vehicle and the code that comes up reads "Misfire Hardware Monitor". Also, every now and then when I start the truck (cold start @ 15 degrees Celcius), the engine RPMs take a second to jump up to cold idle speed. It's hard to explain, but it almost feels like not all cylinders fire up at the same time. It lasts for 2 seconds and then all seems fine. I haven't had it checked out yet, but does anyone have an idea what could be at fault and what I should tell my mechanic? Again, it doesn't do it all the time - regardless of the temperature and conditions outside. Thanks, Andrew