balance

I would definitely advise against getting a stronger spring. That will not make the balances last longer, and indeed, might make them harder to operate: if the spring is too strong then you will have difficulty closing the window. In extreme cases the window will even open up on its own.

Before we go any further, can you confirm the exact weight of the sash? Can you also confirm that your old balance is identical to the Series 385? Even small differences can make a big impact. Seeing one of your old balances (assuming you still have them) might allow us to determine what the issue is.