1996 Nissan Sentra Engine temp. guage fluctuates between no

I recently (like yesterday) had my radiator cap replaced because the old one was defective and boiling coolant out the top. Everything was fine until a couple hours ago, when I noticed my temp guage going from about halfway between cool and hot to almost all the way to the top. In about 15-20 seconds, it would go back down again, only to go back up in another minute or two and repeat the whole process. I've owned this car since 1997 and never had engine cooling problems until now. Any idea what could be causing this?