Coolings system help

[scotty64]

So I have a brand new to me truck. It can be seen here:
http://67-72chevytrucks.com/vboard/s...d.php?t=467035

I drove it home (6 hours) and the temp stayed about 180--so I assume it has a 180 degree thermostat in it. Two days ago, the temp gauge climbed to about 220 and then back to 180. (As background, I know that the radiator has a pinhole leak in it. When I checked the coolant level in the radiator it was low, but the core was not quite exposed). Anyway, I topped it off and all was well.

Tonight (trying to recall my high school auto shop class) I took off the radiator cap, started it, and watched what happened as it heated up. Here's the deal:

Between cold and 180 there was no action in the radiator at all (no fluid movement). Between 180 and 220 the radiator "burped" multiple times (meaning that the fluid welled up out the the radiator neck and onto my garage floor). Then, at about 220, the temp started dropping, the burping stopped, and the fluid was clearly circulating (and it started steaming). I also fished out some black RTV (silicone) that was floating in the radiator--kind of a semicircle like it had been used to seal something.

I guessing that this is a "starting to fail" thermostat? Any other thoughts.

Thanks.

Scott