I'm cursed with faulty instrument jacks... they generally lead (no pun intended) to me swearing my head off when I'm playing. Sometimes it's just a case of a loose wire which can easily be remedied with a bit of soldering.
Sounds like your problem is on the amp end. But if it is on the instrument end, a short term solution is to use a lead with an angled connector and wrap it around the strap so that it doesn't jiggle around too much. I suppose you could try a similar approach on the other end if the problem lies with the amp jack.
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