I can relate to your problem although not with the fruit van but with some of the Dapol normal wagons which I too have had derailment problems. I have normally found that it is due to a very slight twist within the chassis which in many cases is caused by a badly fitted weight which on inspection has become bent which in turn has been as a result of the 'over sized' retaining screw being badly fitted. I have found that the best way to cue the problem is to remove the weight and add my own in the form of lead shot glued within the van body, this also gets over the other problem that Dapol wagons that on the whole they are far to light in the first place.