I have a 1993 wagon 940 series. I noticed after a time sitting at idle, the temp gauge will go past half way position and start to show car overheating. I put in a new relay, that didn't kick fan on. I then put in a new thermal switch in radiator, that seemed to fix it, fan would operate and click on and off. Now 1 week later, same problem, fan won't kick on, UNTIL I rev engine higher by giving more gas, then fan clicks on the way it should, and shuts off. The fan should be starting and stopping by itself, anyone have an ideas on how to pursue this?

thanks.