Lack of heat?

For some time now I have had a lack of real heat. I changed the following, heater core, radiator, heater return hose, thermostat twice (after market ) temperature sender unit. I have even tried blocking off part of the rad. At idle it does warm up however as soon as you go any distance it goes back down to the o in normal on the gauge.
I realize there is a bypass valve in the thermostat housing but it is not stuck as I even got a used one and cut it open to see how it worked.
This car used to be ok . Any ideas?