engine revs but does not accelerate as well, esp. in lower gears, but also in 3rd, 4th and 5th.
If your foot is off the clutch pedal, and the engine revs without acceleration correctly, you probably need a new clutch.
The problem might be with a slipping clutch. Try putting the transmission in 5th gear and then slowly let the clutch out. If the engine doesn’t stall when doing this, the clutch is worn out.
Yup! This surely sounds like a badly slipping clutch.
How many miles are on the odometer of this mystery vehicle?