Your combination of symptoms is confusing.
- Shuddering - usually caused by contaminated clutch or flywheel. This can happen if they did not resurface the flywheel (clean all the old gunk off) when the clutch and pressure plate were replaced, or if driving technique caused clutch to get hot and transfer material to the flywheel.
- Loss of power (acceleration?) - usually due to clutch slipping
To check for slipping clutch:
- Find a long uphill highway, as steep as possible. At highway speed (about 45mph+) put the car in 4th and floor the accelerator. If your rpm rises proportionately faster than your road speed, the clutch is slipping.
You may also have a problem with the slave cylinder or linkage on the hydraulic clutch. If the clutch master or slave cylinder loses its seal, the clutch may not disengage completely when you press the pedal down. The master (on upper part of pedal) can be changed fairly easily, but the slave requires transmission removal.