e39 v8 water issues

can someone help, i have a v8 3.5 and it keeps losing water, i think its the head gasket, ive heard about k seal does it work, temp fix. i want to do a complete engine build over the winter, symptoms are, top up in the morning with water, lumpy tickover,slight missfire then when water drops below half on the expansion bottle it runs sweet as a nut, pulls and pulls like a train, it did blow of the top hose and overheat.

You can do a pressure test on the cooling system or a leak down test on the cylinders.
If its leaking coolant into the cylinders you could pull the plugs, the one(s) that are nice and clean are the one(s) that have the head gasket leaking.
On a friends car when I pulled the plugs steam came out of one of the plug hole which was all we needed to know that his head gasket was shot.

hi peeps just to let you know bought some k-seal and put it in the water system, will let you all know how it performs, i know its a temp fix, is there a rebuild kit for these v8, because when i do the head gaskets i will also put new big end bearings in and rings in too,

Hi peeps its been almost a week now and water level has stayed the same no loss will keep you informed. K-seal does what it says on the tin. so far so good, ill empty the system next wee and put some anti freeze in and more k-seal in, bleading the system was a piece of cake. (put heater on full fan on 1 top up,undo bleed screw, top up, i have a pump on right hand inner wing, must be to do with the pre warm for winter,top up till no bubles hey presto system bleed start engine bring upto temp and great)