Fluctuating speed – motor off

Hello together, a problem that occurs occasionally with my Forester: One at idle times moderately fluctuating speed. I think it only occurs in gas operation (LPG). The variation of the speed indicator sometimes moves between approx. 1200 and the bottom line, whereby in the lowest position the battery usually lights up briefly before the engine catches again. Several times it even went out in such a situation, while the ride(!) Interestingly w ar that every time at the same place of my morning way to work: on a longer run-off track that leads to a crossing, whereby I first roll with gear and then when approaching to the intersection the clutch kick and brake – zack engine off. Was already twice in the workshop with it, where always something was “set” at the gas nozzles – but an improvement was not or only noticeable for a short time… What also stands out: Even if the fluctuation was just ni At the last workshop meeting I was told that this was “normal for cars with overhead camshafts” – is there anything about it or was I told crap here so that I could keep quiet? For tips on these problems I would be really grateful. Greetings N