Manual transmission shudder

Try this, hold foot on the brake pedal. And up the RPMs to maybe 3000, while in 3rd or 4th gear, with the clutch pedal down to the floor, car is stopped and in a safe parking spot, slowly let the clutch pedal up and slightly load the engine only for 3 seconds or so. Then push pedal to release the clutch. Do this maybe 2 or so times. This has worked for me many times.
This is to slightly heat up the surfaces and bed them a bit, and yes its slipping the clutch to do so. Don't over do it though.

Your not releasing the clutch all the way doing this its just a slight slipping you want and not more than 3 seconds.

Sometimes oil can get on the clutch surfaces and this helps burn it off. Did you have any oil leaks? Engine rear seal or transmission input seal? As that can cause chattering.