1994 Other Toyota Models Starter motor Alternator or batteri

In January the starter was replaced. All was well. Shortly after both batteries were replaced. Everything then worked fine. Last week the starter motor started turning the engine at about half speed and this is intermitent. Sometimes faster, sometimes slower. Took it to the garage that fitted the starter(24v starter) and they can't say at the moment whether its the alternator(batteries not going flat) or whether the starter motor is faulty. Going to France in a few weeks and don't want to break down over there. The vehicle is a Toyota Hilux Surf 3 litre but its not listed above. Would appreciate any suggestions.