Also worth noting: never discount the possibility due to lazy thermostats. Recently, I had my fair share of frustrations on my wifey's 93 Camry Vee6, with very little heat and somewhat fluctuating temperature gauge readout (it would register 1/2 way on the scale when idling/running down city streets, drops down to about 1/3 of the scale when flying down highway). While the thermostat seems to be fine during initial warm up phase (it would cut off the cold coolant from entering the block successfully during warm up), nonetheless, it was not meeting my expectations). Turned out that the thermostat became lazy over time (it's the OE one that lasted over 210kms to date), and replacing it with a brand spanking new one from dealership (OE-possibly made by Kuzoh Japan, 82C standard temp0 fixed the problem nicely. As a result: car warms up nicely, register properly on the temperature gauge, never miss a beat when flying down the highway (register 1/2 way on the scale), and most important of all: I get some serious heat inside the car in the midst of winter!!!)
Whenever in doubt, check the thermostat first.
Q.