We are talking end to end rather than side to side arn't we?
Best guess is the engine is still trimmed up (out) too much. The engine is lifitng too much boat out ot the water which is then unsupported at the front. Front drops and the engine then lifts the front again.
Trim the engine in and plant more of the hull in the water which will give a stable boat. 100 hp on a 17' boat should cause problems.
You could try fins on the engine, but that would only mask the underlining problem, better to sort that out.
He hasn't left the trim / trilt pin in has he ?
T
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