F150online Forums - 2006 Forsd F150 XLT 5.4 triton false positive temp reading

purchased used work truck for a work truck. had some issues fixed before i bought it. i took it for a test drive and the temp spiked. mech looked into it, could find anything. drove it again. spiked again. took it back to the mech and he finally found the issue, the temp sender. cool, swapped that out. a week later after purchasing the truck it happened again. driven 1000 miles since i purchased. the dealer is working with me to fix this problem. i was driving down the road glanced at the gauge. it was normal. then the alram tone turned on and high temp. can to a stop light and turned the truck off and back on. went away. came back shortly down the road. then knowing that it was false continued to drive it. all of a sudden it returned to normal.
list of issues noticed
1. goes through coolant like no other (gallon a week)
2. no coolant in oil or oil in coolant
3. pressure build up in coolant tank when hot ( i know this engine is a pressurized coolant system, but it is more then normal)
4. steam is coming from exhaust after start up. ( even on hot days, here hot is 80*F)
5. no service light
6. no coolant on ground before/during/after driving
7. no loss of power in engine performance.
8. coolant is 50/50 mix coolant/water
9. odd bubbling sound from dash in cab. cab does not smell of coolant.

where to look next? read some posts, instrument cluster? ECM? Short to ground? coolant flush?
truck has 172000. looks like original engine.
no other problems then that.