BMW battery light flashes & car jerks

The battery light flashes for a brief second & the car will **** slightly then is fine. I have had a new battery & alternator but the problem is still there.
Called out the AA who advised it is an electrical fault & could be the engine earth wire.
Also had a diagnostic run on the car & the independant garage advised that it could be the timing which is very expensive to repair & can only be done by an approved BMW garage & no gurantees it will solve the problem.

Any ideas???