Hi Marv, how's it going?
Sounds like possibly our ol' pal the mono-water valve insert. it's a little tougher to check on a 126. It's between the 2 firewalls, more on the passengers side. Has a 2 wire connector, a silverish button in the center, and it's held together by 4 slotted screws. Remove the connector, relieve pressure in the cooling system (remove pressure "radiator" cap) then remove the 4 screws. Pull the guts out by the silverish button. You need to carefully inspect the rubber bellows by carefully pulling on the center rubber seal, your looking for a tear in the bellows(the outer rubber section, below the smaller rubber seal). The bellows is available seperately as a "mono-watervalve repair kit". If you need info to replace it, let me know, but it's all pretty easy.