Starting a BB Duster project

I've been driving the car for a few weeks now and the cooling system has me a little puzzled. Around town it never gets over 160 degrees even though I have a 180 thermostat in it. I drove it on the freeway the other day and the temp went up to 170 but dropped back down to 160 when I got back on the surface streets. The outside temp has been cool around here, mostly in the low 60's so I probably need to wait for some warmer weather before I know what is going on. I just can't figure out why it doesn't get up to temp. I suppose I could have something wrong with the thermostat. The radiator and hoses get warm so I'm pretty sure water is flowing thru the system. I was thinking that maybe the thermostat was stuck open but it warms up fairly quickly. I suppose I can pull the radiator cap off and fire up the engine to see if I can tell when the thermostat opens.