97 E320 cooling problems

I recently notice that my '97 E320 was running warmer than usual. On the advice of a friend I changed what he thought might be a "lazy" thermostat. The temp runs cooler sometimes and then will run a full "click" higher at others. There's no way of predicting if it will run hotter or colder (weather doesn't have any effect on it).

The last time that I had a problem like this (believe it or not, it happened with a car that I owned) it was on my '89 BMW 525i. That time it was a bad headgasket.

Any ideas?