[LeRich]

Chasing leaks is a pain but at least if you're loosing coolant you can be satisfied it is going somewhere!

The cooling system is weak and has numerous common failure points. The IMG is a common one - it's worth replacing even if you're not 100% sure it's your cause. Water pumps and radiators leak; the elbow at the top of the rad can fail; the expansion tank can sometimes fracture (not overly common); the thermostat housing can leak; any of the pipes can split.

It's worth considering the heater matrix too as these can leak, although at your rate of loss I would expect this to be obvious through the odour and steamy windows.