I've got a 3.0L which has 138xxx km's on it. Bought it as a project knowing it had engine sludge problems, and a collapsed lifter. Recently replaced all lifters, push rods, rocker arms, camshaft position syncronizer, head gaskets, intake manifold gaskets, exhaust manifold gaskets, valve stem seals, etc... I noticed that I am losing about a litre of coolant every 150km's or so. Opened the oil filler cap, wiped it off and found green. Obviously coolant in the valve covers. I'm wondering if I've missed something? Did something wrong? or faulty parts. I've heard of FEL-PRO gaskets commonly being faulty (I used Fel Pro). Could anyone tell me what the issue most likely is before I rip the engine apart again?
I used RTV Black on the edges of the rubber seals which are with the intake gaskets (as noted in the haynes manual), and granted that my torque wrench is functioning properly, I torqued the heads and intake manifold to the proper torque specs as listed in my Haynes Manual. However, I did have the third head bolt break, which lead me to remove that head again, and remove the bolt. There was some coolant splashed on the head gasket (from the bolt breaking?) which I dabbed dry with a clean rag. and re-installed. I did notice that the gasket with coolant spilled on it, no longer felt "sticky" as it did before.
ALSO, had an o2 sensor code show up recently after rebuild and I know burning coolant can cook an o2 sensor very fast.

I want to fix this before I sell it, so some other poor sap isnt stuck with this issue.

Any help would be EXTREMELY appreciated! :)