Replacement battery, of same part number reboots my phone?

Hello,

I have an early iPhone 3GS that shipped with a #616-0431 battery. I ordered a battery from directfix and tried to install it as im very technically experienced. It was the exact same part number that my phone had. Opened it, installed it but then my phone would reboot about every 2-5 minutes. I kept the battery i took out and and yes im using iOS 4.1 (8B117) and MC145C.

It works if I put the old battery back so I sent the battery back and they said its fine but refunded my money anyways. I see so many people with issues with batteries and iOS 4.x and its hard to weed out what applies to me.

the reason I originally wanted to replace the battery is the phone is 1.5 years old and since 4.1 the battery life is crap and I have to charge it daily to use the phone.

What battery do I use that wont cause things like Error 29 (which i didnt get) I dont want to replace a perfectly good phone and Apple isnt very helpfull.