> found cracks and welded them before rebuilding it
Unfortunately, I'd say that almost guarantees that one of the welds didn't
hold. Welding cast iron is very tricky, and the result is almost always not
as strong as the original casting (which wasn't strong enough). Even if the
weld itself is strong, the welding process introduces stresses into the
casting, which add to the regular stresses of heating/cooling in operation.
But it could be just a leaking head gasket. I've had several of them that
wouldn't even show up as bubbles in the coolant, at idle. The gasket only
leaked under heavy load (where cylinder pressures are highest).