I've had that happen to me before - chain would skip whenever I put big effort into pedaling. It turned out it was my bike frame and not the derailleur, nor the chain itself. The seatstays or chainstays had fatigued enough that it would not facilitate hard pedaling efforts no longer. This was when I was riding my Trek 2100 road bike - carbon tubes with aluminum.

So it may not be the components at all but your bike frame finally giving up.