Nimh will last longer but today's problem is with the charger. Most chargers are quick charge, Nimh needs a trickle charge (overnight) for full effect. Other than that, don't waste money upgrading unless you are using the correct charger. I've successfully rebuilt with the solderless connectors.
I've been ordering rechargeable battery repair kits for my power tools for the past year or so. Patience was required the first time around, but have to say it's a breeze doing the replacing now. You should do okay with yours.