Delay in ShutDown.

The past couple of days my truck will not shut down immediately after I turn off the key. I turn off the key & all the accessories shut off but the engine will keep running for about 5 to 10 seconds. It will not do it all the time but seems to be more frequent with the hot humid weather we have had since last thursday. As a matter of fact it started to do this last thursday afternoon. Anybody got any suggestions?
It really strange to shut down & get out of the truck & the engine is still running. /ubbthreads/images/graemlins/shrug.gif /ubbthreads/images/graemlins/frown.gif /ubbthreads/images/graemlins/crazy.gif

Reg