My "89 325i temp gauge will jump (from 1/4 to 1/2 way point) to 3/4 then jump back down. I know the coolant is not running hot that it is in fact the gauge. Also I am new at all this, what does the 325 mean and the "i" as well. Also, my check engine light will come on from time to time. It eventually goes out, could be 1 hour driving time or 3 minutes. Is this a bad o2 sensor?