Ditto. The K18 was a fabulous engine, but not without its' problems.
Can you see any leaks anywhere ..?
Check around the inlet manifold and also the transfer box. If not, find a trustworthy garage who know the K series engines and get it tested properly. They should use the ' Sniffer ' method and also by pressurising the cooland systen to approx 16 psi and checking for leaks. If it does turn out to be the head gasket make sure that they check the cylinder bores thoroughly as K's are known to 'pick up' and cause damage ( you'll see which cylinder(s) are leaking by the clean piston crowns..
Eddie