Just to check - when you've sat in traffic for a while, does the gauge get up close to 90? If the gauge is low, even after sitting in traffic with the fans on, then the gauge/sender is knackered. If the gauge gets to 90 in traffic, then goes way down when you are moving, then it is the thermostat.
Normal behaviour is up to 90 thereabouts quite quickly - I think normal temeperature is about 85 or so. So 5 minutes after setting off, the car should be hot. If you sit in traffic, the needle should go up a bit, then the fans come on, and it should go down a bit. As soon as you get moving, it should drop just below 90.