Sounds to me like the common problem of the clutch slipping.
Usually you can see this in 4th-5th gear. If you are in 4th or even 5th gear and simple get into the throttle you will notice that the RPMs jump around and there will most likely be a slight vibration.

If this is the case its your clutch disk not being able to hold the flywheel. This is normal happens with the friction material is almost gone.

So you'll be needed a clutch service here soon (new disk, pressure plate, throwout and pilot bearings.