Thermostats (and thermometers, for that matter) seem to have a finite lifetime. Glad you detected a problem with yours and switched to a more reliable method.
I've been roasting for decades, using the high initial temp for about 15 minutes, and then a much lower temp for the duration. The idea is to use the high initial heat to sear the outside and help lock in juices, and then the lower heat to avoid toughening the meat.
Usually I roast this way with chicken and turkey, even when I'm doing a rotisserie, which tends to sear the outside anyway. The exception would be a counter top chicken roaster that doesn't have a thermostat. But those seem to come out OK anyway. In any case, I usually throttle back the temp to 325 or lower for the longer cooking time, at least for poultry.
For pork, usually I get a pork "butt" (shoulder blade) and slow cook in a crock pot with a temp probe to an internal temp of 180F. The internal gristle gets softened by the long slow cooking and actually tastes sort of sweet. I understand this is similar to the method used by old time pork BBQ roasters, who cook the pork for hours, slowly, until it reaches the high internal temp, albeit in a big roaster, not a crock pot.
A pork rib roast is probably more tender than a blade roast to begin with, and doesn't require the long slow cooking method.
