I know this is an old post, but I found a resolution, so hopefully it helps others.
After using jl_cmdr to fully wipe the phone and upgrading it to the latest 4.5os (at the time), removing the microSD card, swapping SIM cards...it still happened. Next up, new battery. found one on amazon.com for under $10, brand new, authentic battery
the phone no longer randomly restarts or powers down during calls. Now the phone is like new
new battery, new OS... all her friends with the Curve are jealous because she has the new OS and they dont :p