once warm, won't go into or stay in 3rd. or high under mod -heavy throttle

What is your question?

Manual trans, once warm, won't go into or stay in 3rd. or high under mod -heavy throttle, (as if getting onto highway) as well the throttle all but stops, lowering the RPMs. There is no grinding and if shifted under low RPMs (throttle) the vehicle will shift through the gears. ??? Mike