Coolant spraying out reservoir & temp up & down

Okay, 1961 Chev Apache Pickup with 75,000 miles.
When I first bought this truck its aftermarket temp gauge would heat up when first starting out to 250 deg, hold for a few seconds and quickly drop back down to 180 and stay there. Figured it was a bad thermostat, so replaced that. Now problem has changed, so now temp will rise and then drop to normal for a while and then rise again and drop and so on and not necessarily at exact intervals. In addition, upon occassion when I check the engine it is obvious that coolant has sprayed out of the reservoir dropping the res. Level way down and even dropping the rad level down and inch. From my research it sounds like two options are a bad rad cap not maintaining pressure and letting coolant out res. Or much worse a head gasket. I get absolutely no white smoke, I have no foam in oil and when running the engine with the cap off, I am not getting bubbles, which I think are all signs of head gasket problems. Need opinions on whether its one of these problems or something else. Thanks.