25 Litres is only 12.5 a side. The Extra tanks are very flat and the wing tank feed is prone to sucking air at low fuel levels. The fuel will easily drain from one side to the other and any slight bank or side slip will uncover the feed on the upwing side causing the symptoms you describe. The same thing can happen in a descent when the fuel moves to the front of the tank. It has happened to me several times. This is why the POH says centre tank for T/O and landing.The ground problem is almost certainly due to vaporisation in the injector pipes which are sitting on top of hot cylinders with low cooling flow. It can also happen on hot days on the second flight when the fuel has had time to heat soak on top of a hot engine.