Car is rev-ing and the odometer goes to 6

This problem does not occur all the time and only happens when I start my
car. Doesn't matter if it is cold or hot or how long it has been
sitting.
It has a mind of its own. I was wondering if someone could tell me what
it might be contributed too? I was orginally told that thehoses were not
tightened so i had that done but it still happens. Help?