If there is water loss internally, there are a few ways that can happen.
Headgasket.
Inlet manifold gasket.
Crack in head, block or liner
Liner seal at the bottom compromised.
The engine light indicates it finds its way into a cylinder, and you have misfire,
I think we can thus safely ignore faults in the cooling systems and outside leaks as the main cause.
I would pressurize the cooling system to gain more information.
I would also just like to stress, that the standard torquing of the head gasket will give too low values, and that water loss can easily come from that.
And, worry about all the other add-ons discussed only when you have found
the root cause of your problems.
I include a link.
http://www.the75andztclub.co.uk/foru...d.php?t=214880
of special interest note the torque procedure discussed in post #34.