Why does my SL5 arm jump up after a few seconds?

Pierre Lurne linear SL5 arm keeps jumping up after only a few seconds. I tried the following: cleaned the record, cleaned the cartridge, increased the cartridge weight to slightly higer than allowed (1.6g. instead of the maximum 1.5g), cleaned the arm carrier, played with the clutch in every sense, arm belt seems to be all right. Level left to right spot on, level front to back out by a few millimeters, and shall solve that problem soon, however, experts tell me that back to front difference level of a few mm. should not have such serious consequences.System: TT Audiomeca J4, cartridge Von den Hul MC2. All was reasonably ok for a few years, and seemed a happy compromise between the hussles and costs of an air supported linear arm, and a truly linear arm, but simple and relatively inexpensive (at least when I bought it, many years ago). I wrote about the problem to Pierre Lurne, "the father" of both the arm an TT, however, he seems to be a one man firm, and is probably overwhemed by the amount of post. I therefore do not expect a quick reply from him, even assuming that he were to care about this. The problem for me is that I much prefer the linear arm, I tried an SME V no less, and still prefer linear tracking. I should be grateful for any thoughts or advice people might have.
Pierre Lurne has extremely promptly, politely and comprehensively replied to my e-mail to him, and there is every chance that his advice will cure the problem. I shall shortly report back to this NG what the outcome is, just in case it might help other users of the SL5.
And now for the anticlimax: after an e-mail from Monsieur Lurne, I realised what the problem might be, and simply washed the belt with warm water and washing up liquid, cleaned a little the pooleys, etc., and the arm works again as wonderfully as it did without fail for the last 11 years, since I bought it. The SL5 is simply a superb linear arm, which costs only a fraction of the price of the great names, yet has all the advantages of linear tracking. In any case, good "hygiene" seems to be a requirement of all linear arms, no matter how exotic or expensive. By the way, I am not connected directly or indirectly with Audiomeca or Monsieur Lurne, and have never been there, nor met him. Now, if you are familiar with the SL5, I would be interested to know what cart you would marry it with.