De-chaining st the front is often caused by misaligned front mech, ensure it is rotated parallel to the chain. Check for any unusual bends in the metal that were not there when new.
At the rear it may be due to a bent hanger, bent derailleur, work jockey wheels or the hi and lo limits of your mech are not set correctly.
First place to look is the high and low limit screws on the rear mech. The upper jockey wheel should line up with the big and small sprocket. If it overshoots, you need to tighten or loosen the appropriate screw. I can never remember which way but it is evident when you start rotating the screw, which way the limit is moving.