Since fixing mine, it does still very occasionally drop to low to mid 70s, but very quickly returns to 80ish again. I really don't know what else I can do, or whether I should even try, this might just be a trait of some Sigma engines. A number of thermostats in, complete coolant flush (twice) as I lost everything when I changed the last thermostat, new temp sender, checked water pump, pulleys and aux belt (fine), check for leaks (none), bled and re-bled system. I did have an issue with thermostats sticking from what I believe was radiator sealant in my system to start with, but my symptoms greatly reduced after the above noted work.