btroms

So i thought my trans was bad because I kept losing first gear. It would randomly go out, with no warning. I would wait a few minutes then try again and i would make it maybe a half mile before it would quit again. It sat for two years. I tried moving it to a friend's house to put a new trans in it and it drove 10 miles without a problem. Haven't drove it sense. Any ideas? Maybe a sensor or just a bad trans?