3 possble causes, all are real cheap so just do them all at the same time. First change t-stat, change the cap, and put the front of the car on jack stands as high as you can and run the car without a radiator cap on untill all the air bubbles are released. I can guarantee you its your t-stat though.
Well first let me ask, is it overflowing out of the overflow bottle, or the radiator itself?
Similar frickin' problem with my 5.0. Just replaced the t-stat and put in 3row alum rad. Has a Vortech S/C. Motor runs and idles fine. No smoke out tailpipes. No apparent coolant in oil or vice versa. Overflow isn't overflowing. Crazy pressure in new top hose. Watched the temp creep up over about a half hour drive. Popped the hood, no leaks anywhere, but the gauge was pegged.
** Pump bad? Gaskets? Doesn't run rough at all and has no apparent power loss. HELP