I recently had a similar problem too. The previous owner had a head gasket job done to my car just before I purchased it. Come to find out, he chose to go the cheap route and just clean the head and not take it to the machine shop. His mechanic ended up using two head gaskets to seal the head to the engine block.
I ended up with the same problem as you losing coolant slowly at first then quickly then slowly again. Car ran a bit rough. No apparent white smoke out of the tail pipes. Temp was a bit high too.
My mechanic did a pressure check and found it slowly losing pressure. No apparent leaks under the car. Ran it a week more then I found a little oil in the coolant. Sure enough it was leaking into the cylinder. The edge of the water jacket was corroded enough not have a solid seal even with two head gaskets and started leaking into the cylinder.
I ended up purchasing a refurbished head and now everything runs fine. Temp is fine, coolant level is stable and car has a lot of pep.
Hope this helps.
1987 260E Charcoal Grey 157,000 Miles
1987 260E Forest Green 120,000 Miles