Mocal Oil Thermostat problems

[dmarv]

The previous owner of my 76 911 had an aftermarket oil cooler added to my car when he had the engine rebuilt. When I purchased the car, 15 years ago, the cooler and thermostat work great. Oil temps never got above 200 even on days that were 100+ degrees F with the A/C on. After a year of ownership, I noticed that my oil temp was climbing to 220+. I checked the thermostat and realized that the thermostat was not working properly. I purchased a new one and installed it. It worked for a month and then the same problem. I got sick of it and didn't drive the car very much in the summer time. Fast forward to this week. I decided to see why the thermostat wasn't working, so I removed it and took it apart. I found the o-ring that seals the plunger and closes the by-pass had come off the plunger and was sliced. I replaced the o-ring and put the thermostat back on. Problem was fixed. The thermostat seemed to be working again. Last night I was driving the car and noticed the oil temp was climbing to 210. I checked the thermostat and it is not working again! It looks like I need to buy a new one. Has anyone had similar problems with Mocal thermostats or am I just unlucky?