Stalls on cold start

6 cyl Two Wheel Drive Automatic 145000 miles

When I start the truck (2.9L engine) after it sets all night it will run and idle as it should but as it idles down it will stall in just a few minutes - I re-start it and let it idle for a few minutes it runs very well most times but at times it will stutter and pulse(as I drive it) but if I pump the gas once it stops doing that - when the temp gets to normal it runs normally - I've changed the plugs and the visible gas filter shows the fuel is flowing ok - do you have any suggestions as the cause of this - Thank you for any help you can offer - Lee.