if your letting the clutch out and the motor is free revving like its in neutral then maybe your clutch is almost toast. either from age or abuse. how many miles are on the car? ever had the clutch replaced?
im assuming since your downshifting when this happens that you can get the car moving without any slipping?
and try this when your cruising at speed. put the car in 4th or 5th gear and punch it. if the car reves way up almost like its not in gear then id say its possibly time for a new clutch.
any extra info you could give would def help with the E-diagnosis.