You guys sure love to spend money. I almost never see coolant leaks from 103 head gaskets. It is way more likely that you have the heater hose o-ring leaking. A hose could be leaking also but they usually aren't that affected by temp; although pressure increases all leaks.
Find the leak, don't just start spending money. I sure would like to be the one doing all the wasted head jobs on this board.
Bosch Master, ASE Master, L1
33 years MB technician