I have a 1993 Legacy Sedan 2.2 with a block heater. I plug it in on cold nights, otherwise no way will it start in the am. Also on cold days, I will start it about every 2-3 hours if I can get away from work. Most often it's fine when I do that. I have found that, like yesterday, it won't start after 2-3 hours. What causes this to freeze up fast? Ultimately, I have to have it towed home and plug it in. AAA has paid for itself. When I finally get it to start, I really have to force it. Could there be another cause for this problem?