Definitely sounds like it is time to replace your thermostat. This is tough to do with the fan in place. You can make the tool to hold the fan pulley while you loosen the fan if you have a drill and a grinder. Just find a picture of one on line and you will see how to make it.
Don't forget that the big nut that holds on the fan is left-handed thread!
If your car still has a black plastic thermostat cover, don't try to re-use it. They shrink and warp with time so you cannot re-use them. Either get a new plastic one or get an aluminum thermostat cover.