Hi Lowflyer,
I had a similar problem earlier this year, I was all set to buy a new battery when Keith from BHM called me and suggested cleaning/tightening the earth from the gear box to the chassis.
This cured the problem and saved me the cost of a new battery.
On my car the earth is found under the bonnet on the near side and runs from the bell housing to the bulkhead and is very easy to reach..
Good luck and hope this helps.
Rob...
