i had a similar problem. my a12 in my 1000 ute blew a head gasket cause it would leak water into the cylinder and when it spat it all out it would run better. i also had a vibration but that went away when i fixed it.

BUT...
for a really bad story that u should not follow read on.

i got of my ass 1 day to change a head gasket on me a12 which had been over heating in 40 degree heat (c) for a couple of months. when i ripped of the head i found tha gasket was stuffed (alot stuffed) so i got out me wet n dry cleaned the face of the head with it and the block, replaced the head gasket $13 and inlet gasket $5 and used the original rocker gasket wacked the head back on without adjusting the rocker arms or torquing the head (just used my ratchet) and i swear its never run better. ps: dont try this at home ITS DODGY