Not wanting to shift into hgher gears

We bought the car off of our friends about
six months ago and the car does not want to
shift into a hogh gear it will sometimes go
into second. This is where it gets weird  
when you drain the fluid and chamged the
filter it went and drove but in a couple
minutes when it got warmed up tho it
suddenly would only stay in first we have
cleaned the Valve body and I am wondering
if it could be the solenoid pack getting hot
and not wanting to shift or if there is some
dirt getting back into the valve body or if the
solenoid pack is getting too hot.