Yea the battery is just probably degrading. Nickel metal batteries don't last as long as lipos do. When I started out in RC I would have to get two new nimh batteries after every year as the runtime and capacity of the batteries would go down.
What I would do is upgrade to 2S lipos as they are...
How old are the batteries?
How many cells?
I used to run 7S nimh packs in my slash and rustler a few years ago. Max voltage of those are around 10.5v. If they are 6 cell, max voltage of 9v.
From what i seen, after a year of owning nimh batteries, the cells degrade and don't last a long time...