I have always used Large-Large+2 and it's always worked for me.

Before you go changing derailleurs, check these conditions. Ensure that the cage is not at the end of its limit on Large-Large, and isn't causing the chain to double back on itself in Small-Small. If both of those are true, your chain length is probably OK.

What this sounds like is maybe a weak spring. What exactly do you mean be "sagging"? Does it just seem that the spring doesn't have enough tension? Is the spring even engaged correctly?