I have a similar issue, and it's obviously not as noticeable on a warmer day as the ambient air is of a higher temp, as such it warms the engine faster. I suspect that the thermo is busted/stuck open and would like to check and replace as necessary, has any seen/done this process? I'm looking for a tutorial. I can drive from my place to work (around 20mins) after letting my car idle for 5mins and the temp guage will only J'just' be scrapping to the 1/4 mark, it usually sits a bit under half when it's nice and warm.