I'm having this issue on my Takamine GD93CE.

Mid play the volume will spike significantly and feedback somewhat terrible, despite the volume on the pre amp already being set to full.

I have ruled out it being a problem of cables or effect pedals and I'm fairly sure it isn't the amplifier. 

A technician suggested it be a battery issue. After changing from rechargeables to regular 9V's the problem subsided for a while but still occasionally returns. In previous electro acoustics I've had no issues of regular 9V batteries lasting a long time. 

Has anyone had a similar issue? Is it just a battery issue and if so, could you recommend a good brand/model of 9V battery to use?

Kind Regards