Horse falling

My gelding aged 17 has fallen, once while someone else was riding him, 3 times with me riding and once in a field.  He goes onto his knees with no warning and falls onto his side.  He gets up immediately and seems fine.  I haven't ridden him for 5 months and he hasn't fallen at all during this time.  I have owned him for 2 1/2 years and the first 2 he didn't fall.  He has had x-rays of his neck and knees, seen a horse physio, his teeth have been checked, a farrier keeps checking his feet and he has had a basic neurological assessment but nothing wrong can be found.  Should I proceed with further neurological investigations?  Is it safe to start riding again or should he be retired?  Apart from this he is fit and well.  I wonder if he is just clumsy and not paying attention but why now?


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