1991 Ford F-150 puttering out

I bought an f150 recently and it has driven very well for its age. Lately though it has been puttering out on me when I am stopped an sit in drive at lights or stop signs. The engine is only idling at 500 rpm when it cuts off. Sometimes it will idle a little bit higher, near 1000 rpms when I have been driving it for a while. Friends have suggested that it could be the fuel filter, or timing problems. I cannot figure it out myself. Please help me to get this classic pick up running right again!