89 Silverado 350: overheating or faulty temp sensor?

[775stepside]

So I just finished the truck and I started the truck and I let it warm up, filling it with coolant as well letting all the air escape. It got up to I want to say 180-185? (Keep in mind I got a 185 tstat) so once I let it idle for about 10 minutes I drove it around the block to the front of my house and noticed the temp guage started spiking all the way to 260 or whatever it maxes out too. I drove it back to my garage and turned it off.

I put the key on engine off and the guage spiked back down to 140 or so, turned it back on and it stayed under 210? Is the reading false on these moonshape guages?