hmmm... maybe your battery just cant deliver what you want it to? or is it a fairly new battery?
don't take this as "i need a new battery!" i'm just trying to think why...
no probs i have my speakers cut out in the front but found out that my lousy attempt to connect the new speakers to the original wiring was so feeble that it fell apart and stopped... and only on right bends.... which is every roundabout! lol
good luck! (Y)