Thermostat is going waving up and down during warm-up

[Donald91tbird]

91 tbird 150k

At first I thought the thermostat wasnt really opening correctly. But since its cold out I gave it a little time to warm up. All of the sudden it starts waving up and down then after a while stays at normal temp. throughout the drive

[jmt]

My 93 did that, and I noticed that it was low on coolant. You might want to check the raidator, and the holding tank.