First of all, read this if you're doing it yourself:
Head Gasket Guide - K Series
Second, was it a genuine cap that was fitted? We've seen a lot of bother with yellow aftermarket caps.
Finally, if your liner heights are spot on (and assuming it's a 4 pot K Series), give the N Series Head Gasket Kit consideration:
N Series Head Gasket Kit
It contains 3 of the parts NAC MG developed to 'fix' the K Series.