I had a similar problem getting mine to work. I had taken a breather vent from the outlet casting on the cylinder head (connection that used to go to the injector heater - which I can do without) to the bottle. Like you , I found at idle it worked fine and squirted coolant into the bottle. Revs up, it sucked air back, which then collected in the radiator and airlocked it. I fixed it by putting a dip pipe into the breather connection on the bottle, so when it sucks back it sucks coolant instead of air. A bit of a hack, but it's worked fine ever since.
Richard
EDIT - here's a link to the thread at the time when I was trying to fix mine:
http://www.madabout-kitcars.com/foru...ead.php?t=1411