02 Ford f250 7.3 starts up, idles fine for a few minutes, then the RPM drops to around 500 then goes up to 1100 and keeps bouncing up and down for a little. Then it'll idle fine and then RPMs fluctuate again. It'll do this over and over until the temperature hits the the bottom line of the guage when it's warming up but it'll actually stall and shut off when it get to that particular area on the temp gauge. It'll stay running once the temp gets above that area, but I have to hold on the throttle til it warms up out of the stalling zone. Any clue on how to fix it or why it's doing this?