If I remember correctly on a 6C there whould be a suction screen for the transmission pump under the floor boards right about where your left foot would rest. You'll probably have to pull the floor pan to get to it but you should see a vertical canister with a square top and botton with a cover held down by 4 bolts or studs and nuts. Remove the 4 bolts or nuts, insuring you don't drop the spring between it and the filter screen. Pull the screen from the housing and seperate the magnetic rings from the filter and clean both items.
No guarantees but this might fix your problem. Anything beyond that will indicate excessive wear in the clutch packs that will require going into the transmission to repair. Good luck.