As I understand your description of the problem, you're not saying that the lower loop is sagging, but that the chain is slipping over the top of the sprocket when you apply power.
If I read you right, it has nothing to do with your RD, and everything to do with the wear age of the chain and sprocket. As chains wear the pitch increases and the sprocket wears in such a way to allow a "stretched" chain to climb outward on the teeth until it's riding high enough to slip forward if pulled hard enough.
You can confirm my theory by putting enough tension on the chain to pull out slack then measuring 12" of chain with a ruler. Since the chain is 1/2" pitch, every pin should line up on a 1/2" mark of the ruler. As the chain wears the play at the pins increases allowing the links to move apart (imagine a freight train pulling out and stretching as each car takes the slack out of the couplers). The error accumulates at each pin, and the rough guideline is that if the chain is stretched much over 1% or 1/8" over 12 it's toast, and in all likelihood so is the freewheel.
If the chain isn't stretched beyond 1%, other possibilities include the chain wanting lubrication, or the RD being slightly mis-adjusted in high (cable and/or high gear limit adjustment).
Start with the chain, that will tell you where to go next.
__________________
FB
Chain-L site
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