Mk 2 Battery and Alternator Problems

Simplest way to explain.

Renault clio
1.4i 8v Sport
2000 model

When idling car is ok ...
turn cd player on ( which has a 700 watt amp attached ) and headlights, the battery and stop light flickers ( can only see this in the dark ).

When accelerating over 3000 revs the light becoms more brighter and is there but faded....

i have gone through 2 alternators already and this is a new part....
Would a new battery fix this??
its currently a 65amp battery..


What could be the problem ???????
Many Thanks
Ash