Failing inner tubes mystery

[billabitbaffled]

Many thanks to everyone for your detailed and clear responses, which are much appreciated. I will try your recommendations and hope for a good result.

[Gazelles]

In my experience tubes are less prone to problems when they are not used in the max. recommended tyre size. So I would prefer a tube that is suitable for up to e.g. 42 mm in a 38mm tyre.