My friend had the exact same issue with his GTX295. It worked fine for a while and then suddenly started freezing. When your PSU isn't sufficient you put a lot of strain on it. Therefor it works at full load most of the time which isn't what it was designed for. Most of them have rating of like 80%-90%. Anyway, like I was saying you put lotsa strain on it and later the caps just can't take it anymore. Even before you posted the minidumps and so on I thought it was the PSU but was too busy catching up some work to reply.
Hope you find something decent for cheap..
