Has the thermostat had it?

Please help me!I have a 1994 Subaru legacy 250t with 200 000kms on the clock. When I bought the car it had just had its head dne and things seemed fine. But after a couple of weeks the trouble started. Firstly I began hearing funny sounds from under the dash then loss of power on acceleration and the light to show what gear im in began working only when it felt like it, finaly the car started running and idling rough and was hard to start.I had all fuel injectors replaced new spark plugs battery fuel filter etc(very difficult on a single parents income) and this seemed to fix most of the problem but not for long!One week later and now my temp gauge is reading off the scale but there is still coolant in the radiator and its not hot steaming or showing any overheating symptoms and the car has lost power and intermittently runs and idles rough again. The heater stopped working blowing cold only and gives an outside reading of minus 25 degrees Celsius. Odd when its a comfy 20 degrees here at moment! What is going on?