Dodge ram 1500 5.2 liter

I put a transmission in and it shifted good
and  everything for 3 days. I'd only went
about a mile or two each day within the
three days. I did not go far in it because I
was hesitated about it . Last night I went
about 6 miles  got it up to like 55-60 it
shifted . I pulled into the driveway and it
just started jumping and killed.  It cranks
right back up, put it in gear, and it just kills
like it just doesn't want to go. When I was
on the road I was trying to come to the
stop sign and it just jumps and kills. It's
like it does not want to go into a lower
gear. It's like it's stuck in a high gear or
something. Can anybody give me any kind
of information on what to look for or what
this might be thank you.