Engine heet up problem

I have 2006 750 Li with 76K on it, all is good except car start heating up when Idle. I did drive about 100 miles without stopping no issue and when I stopped at the end. engine sign sign come with temperature and box burst and all anti freeze spilled. took it to BMW, they replaced the Box (don't remember the name which hold the antifreeze), put new antifreeze, replace the thermostat.
test drive no issue, but sitting idle for few minutes, same high temp. sign.
I took chances, drove back 100 miles back to my home town, no issues
take it back to my hometown BMW dealership to check, they tested about a week no temp sign but they did notice some kind of smoke coming out of exhaust when idle, took the car back, few days no issue.
Now the temp. signal back again and car performance bottom most during high temperature.
Not sure what to do if the problem is there and even dealership cannot find out issue. did someone see the same problem.
Also dealer ship talking about may be antifreeze is mixing up with engine oil somewhere and talking about 8000 expense to fix some engine seals but still no guarantee about the high temp. problem

any help will be greatly appreciated.