Automatic transmission oil coolers

Carol0154

We have a Peugeot 806 from new (2002) and it has always towed well, it is petrol 2000cc.   However since changing caravans the gears play up once we have done a long journey.   The new van is heavier but still within the cars limit as shown in the handbook.
  The garage has changed the oil twice, saying that it is burnt through overheating but no other advice on how to stop it overheating.   We recall that the Club magazines always used to have adverts about retro fitted oil coolers for automatic gear boxes but
we can't find an expert on them now, are they a thing of the past?   What can we do?