03 Isuzu Rodeo 3.2 V6 2 wd w/ 145K long crank time w/ high idle and temp gauge on Cold?

During hot months and intermittently, after driving for awhile then shutting off to go in the store (for example). It takes about 5 secs of cranking to get it started. Once started, the idle bounces between 1800 and 2200 rpm for about a minute then settles down to a high idle of about 1000 (700 is normal). My temp gauge is buried in the cold (I was only in the store for 10 minutes and it had been at operating temp). It s drivable like this but has very little power off the line. Strange thing: I turned the heater on full blast for about 10 sec. and when I turn the heat off, the temp gauge went back to normal, the idle also went back to normal and it started running good again. ANY IDEAS??