2008 Legacy 2.0Diesel overheats

Oke not sure this site still works but anyway, ill tell u a story how it begun. I bought the car 2 years ago, its 2008 diesel boxer 2.0 manual. I drove like 2 months with it when there started a problem, came winter, and the car just didnt warm up. No matter didnthe engine run for 30-60min, it only warmed up during the driving and same thing with the heating, it only heats warm when im driving on the winter but one day this all stopped, it disdnt heat any warm air, we replaced the t-stat, checked the waterpump and stuff , any leaks, didnt notice anything and it was frustating. Then the last thing we did, was the pipes that goes into interior, we used air compresser to on the pipes that goes into interiir radiator and stuff and used the air compressor, a bit crap came out but after this, there started heating warm again, i was glad i could drive now in winter, but it wasnt fixed 100% cuz the engine still didnt warm up while it was just running, oy warmed up when i was driving, and the temp went back low when i stopped and the heat was a bit colder too, weird thing was also this that the passenger seat didnt heat that hot air as the driver seat but i was glad that it worked at least, now like 7months later, in the summer everything was nice, but when i used more power ans idle , fukl gas like 15-20sec (no matter ehat gear and speed” the temp started to rise and as soon as i hit the clutch or release the iddle, temp webt back to normal until one day the temp started going up even with a normal driving and now i cant drive the car more than 30min, cuz as soon as it gets warm, temp start rise, when i stop it goes back to normal and even in these hot days, it heats cold air, i havent let this overheat yet, coolant level is normal, no air or leaks seems under the hood, i read about white smoke (its a diesel) white smoke comes from the exhaust only if the car hasnt been running all day or days and white smoke comes out only a little time when i press the accelaration, if it warms up a little bit, then theres only black smoke. Could it really be head gasket or should i replace the waterpump or there may be crap in the hoses or radiator?