I recently had the head machined and the head gasket replaced due to a
blown gasket on my 1990 honda civic wagon. after this everything was
fine, except for a hose that was not attached properly; the hose popped
off and coolant sprayed everywhere, then the engine temperature went
into the red. I noticed this immediately and then found the detached
hose and reaffixed it properly and filled the coolant and radiator
completely.

I'm finding now that I loose about 600mL of coolant every 400km or so;
after about 1200km the lost coolant is enough so that the engine
temperature will go up if the RPMs stay over 3000 for a sustained
period of time (like on the highway).

I have a suspicion that coolant is evaporating somewhere or somehow;
what happens is that when the coolant becomes low the temperature goes
up when it reaches the air "bubble" and then it goes back down when the
bubble passes. I'm thinking that it might be more coolant evaporating
everytime the coolant to air ratio changes. that is to say after 400km
I'm out 600mL, after 800km i'm out 1500mL of coolant.

Am I way off the mark here? Any ideas what might be causing this and
how to remedy it?