Running HOT, or not

[Sinjun]

Hey guys,
I purchased a 72 C20 2wd last fall, and have been working on it feverishly since. It's comming along, but have a question. Previous owner changed out the engine for a 283, and did a decent job. It appears to have a new radiator, and since I've had it, it does this strange thing... The temp guage will climb slowly all the way to it's hottest point, and after staying there a minute or so will climb back down again to about the quarter mark. I replaced the thermostat with a new one (185) and believe there was some air in the lines, and have been keeping an eye on the fluid level and filling it as needed. Any way, is this dangerous to allow the temp to peg on the high side, and let it come back down to what I believe is normal? What might be causing this?
Any ideas, thoughts, would be greatly appreciated. It's cold out there, and my motorcycles just went up for the winter, and would rather not find myself with an unexpected walk on a cold winters night.
Thanks!!!
-Chris