Perhaps there is a blockage somewhere, maybe in the radiator. Are you getting excess steam out of the exhaust (usually accompanied by a slightly oily residue and even a sweet smell)? I'm guessing this motor doesn't have a cap on the radiator, but rather the system is sealed at the coolant bottle.
If you're careful (emphasis on careful), you could check the hoses when this happens to see if any feel cool. Once you find that, then you should be able to figure out the source of the blockage. Could even be a bad thermostat.
I'd advise to remedy it ASAP so that you don't ruin your motor or cause a head gasket issue.