I have had a similar problem. I was losing coolant (still am too as I seem to have an external head gasket leak) Things I found out while trying to fix it.
First off. I never got all of the air out of the system. As a result it seemed to run hot.
We hooked it up to the OBDII after I got that solved and found that when the fans did come on ( the temp gauge was past the "L") the temprature was still at a safe level. I think it was 189 or so. What I am getting at is the gauge can be very misleading. Now when in stop and go traffic or idling I still hit "L" from time to time. often I turn on the AC at full and it cools the engine back down to the "M" which is where it spends most of its time anyways. I also should note due to the leak that I have the coolant cap is not tightened down much at all. Since doing so the leak is nearly gone.
