06 brm loses heat

251,000 on clock
Small soap suds like bubbles returning to overflow tank constantly. Tested a handful of times for combustion gasses in coolant always negative. Car heat comes and goes on daily commute and coolant light comes on every few days due to coolant being pushed out overflow tube. Added head gasket repair and calmed issue down for several weeks but now back again. I can only assume it's clean charged air from a crack somewhere between charged air and coolant port???
Any input from all the knowledgable folks would be greatly appreciated!

Many of the combustion test methods have to be diesel specific, there just isn't enough latent hydrocarbons in a diesel's combustion event at idle to detect, because they run so lean.

I'd take the glow plugs out, and pressure test the cooling system and let it sit for a while to see where the coolant goes.

Head gaskets are not common on these, and while EGR cooler failure IS common, this is not the method in which they fail. They leak exhaust externally at the changeover flap shaft, causing a hissing sound.

However, if the engine was overheated at some point, then anything is possible.

Is what you mean interior cabin looses heat? Typical reasons for loosing interior heat would be air in the system. This would be noticed at IDLE especially. Fan will blow cold air at engine Idle. If you have not filed the reservoir up to proper level post work it will suck air into the coolant system. Bubbles...Be sure the coolant is at the correct level in the spherical reservoir. Be sure all air is out of the system and reservoir is at correct level..... See Manual for details... Hope that's all it is..