speed sensor is on main shaft of transmission.
in way you decribe transmission problem, it makes me think that some gear clutch plates worn out, and that causes slippage.
you can take your car to transmission shop and have then to evaluate it, they offer that kind of service for free. but don't let them do any work on it because that gone cost you.
if they say that your gears are slipping, you should get "new" transmission from junk yard and have installed by same transmission shop or any repair shop. transmission shop may not like that, because that is minimal earning for them. getting transmission from junk yard is whole a lot cheaper then to rebuild old one.
you can search for your "new" transmission at
www.car-part.com.
it's network of junk yards across north america, so you can get transmission from local place or from canada, if price is right. some junk yards offer one or two year warranty on transmission, so if something wrong you get replacement for free.