91 Civic 1.5 with Sporadic Overheat Issue

I’m working on a 91 Civic and am having a heck of a time chasing down the cause of a very sporadic overheating issue. The car belongs to my sons room mate so I have never been there when it overheats but have ran a ton of diagnostics and haven’t been able to pinpoint the cause so before I tear into it I’m hoping this story will sound familiar to someone. So…..I get a call from my son’s room mate and he tells me he overheated on the freeway. He pulled over right away and noticed coolant had sprayed all over the engine compartment. He had it towed to a local mechanic who proceeds to tell him that the radiator hose fell off so they reconnected it, filled it full of coolant and sent him on his way. Two weeks later he calls me again and says it started overheating on the freeway again. This time when he pulls over he discovers the overflow cap had come off. When he tells me this I’m saying the hose and the overflow cap didn’t just come off, they must have blown off under pressure. I tell him to have it towed to me at which point I check all the usual suspects. No white smoke, no coolant smell in exhaust, no milky oil, no creamy deposits on the oil fill cap, no external leaks and the fan is working fine so I change out the thermostat, re-fill the system, burp out all the air (following the video instructions on this site) and replaced the radiator cap. Car runs fine for another couple weeks and then overheats again. Coolant reservoir blew off again. So, it’s overheated twice on the freeway and once going up a hill so obviously it’s getting hot under heavier loads. This time he drives it back to the mechanic who worked on it first and of course they can’t find anything wrong with it. A week later it does the same thing so…..this time I get a block tester as I’m sure it’s a head gasket but the test is negative for exhaust gases in the cooling system (yes I’m sure, I did the test three times). About the only thing that hasn’t been tested or replaced at this point is the radiator and the water pump. I would think if it was the water pump it would overheat all the time? Anyway, sorry for the long post but that;s the story. Any advice at this point would be welcome.