Overheating

Some days by time I get to the end of the block temperature gauge reading half way blowing ice cold, get about five minutes down the road nice and toasty. Bam now blowing ice cold, temperature going up, turn on the A/C temperature comes down. Pressure blew the cap off recovery tank, bought a new one filled everything to correct levels. Car running hot again coolant went on to recovery tank, but it will not siphon back to the radiator. Going through about one gallon coolant a week have not a leak yet. Still looking. Replaced cap, thermostat, upper and lower hose. I am normally old school small block Chevrolet type guy. When I went to AutoZone and ask for temperature sending unit for the Honda the behind the counter typed it in this handy dandy computer and tells me there is no such thing for my Honda. Do they call it something on these cars? Because there sure is something screwed in to my thermostat housing with wires.