1991 Ford Fiesta Engine Perfoming poorly when turbo Kicks I

Recently bought an RS Turbo but it was stored for around 6months before and the battery left flat, now I have started running it around the battery hasnt been holding its charge and also when I put my foot down sometimes it begins to judder like its not gettin sufficient fuel on boost? The problem stops when I switch the engine off and restart it the problem then comes back if I put my foot down, does this sound like an electrical fault such as a sensor down? Thanks