You have a bad thermostat. A weak spring in it is opening it at much cooler temps than it should open at, thus making your engine run cold (and less efficient)

I ran 175 in traffic all the time, but on the highway it'd cool to 150 or so (or if I started out on the highway, it'd never get above 150). When I replaced the thermostat, this was solved (and also gave me SOME heat, I had none before that - now I've pretty much solved the poor heat I believe (but that was due to clogged heater valves).

The cardboard doesn't do too much when your spring is strong (thermostat is good) - it might help it get to operating temp a little quicker but not really too much so unless you're doing 40MPH+ with light throttle The thermostat, when cool (especialy at below freezing) allows very little to no coolant to flow.