If the drive is slipping in geared mode, the grooved nut on the left side probably hasn't been adjusted properly! This nut has to hold the reaction torque of the drive, means a similar torque as you excert on the pedals has to be held by this nut!

Please check: if you have a mountain-drive or a high-speed-drive, you can see if the housing rotates. If so, remove the left crankarm and tighten the nut properly with 140Nm and a torque wrench! BTW: Did you use the correct cone material? Aluminum for steel frame, steel for aluminum frame! Very important!

If you have a newer speed-drive with steel cover on the back side, you can mark this cover to check, if it slips under load. If it's an older version with plastic cover, a rotating housing isn't visible from outside.