Here is some perhaps relevant experience....
A VX engine, new to me and the chassis seemed to defy all attempts to make it run for any length of time without boiling over. The car was in the hands of come experienced bods at the time, although there were not as familiar with the XE engine as they might have wished. This problem which sounds very similar to your symptoms went on for days, with several different sets of eyes looking at it to see what they might find.
HG issues were suspected. (In fact most things were thought suspicious all the way through this saga!) The only difference to your story is that mine was running a heater. I have also seen similar issues in an XE powered 21.
In both cases the problem was traced to a poorly routed set of hoses around the "submarine" or the "J tube" (same thing different names). Hard to believe after all the fettling that the solution was *that* simple, but it was. Poorly locating the tubes that come off there can lead to a "built in airlock". Its the hardest thing to see as its under your induction and a real wrist killer to fettle. They were'nt the wrong way round, just poorly routed.
Have a careful look and see if there is anything there that might trap air....
FWIW, while on my travels in Germany in 2003 I ended up draining my coolant whilst at MOG racing, Joachim produced a handy little pump - like the balloon pumps one might buy on the high street, with a header tank cap already attached to the end. The pump was used to create some pressure in the system and then I watched as massive air bubbles came back up from somewhere down below when the coolant was forced down into the system....
So, in conclusion, despite several opinions that my HG must be toast or that there was a crack somewhere etc etc, I'm glad I perservered with the plumbing iussues. The engine has been in there now ever since (20 000 miles?) and has not had any HG or cooling issues since...
Vinnie