I had two of these for four years and they did not have this problem. I ran APC 12 X 6 props

I have experienced your problem with a Saito 72 once. I had been using a 13 X 6 and never noticed a problem but changed to a APC 11 X 9 that was in a drawer at home and the engine had a very rich mid range at the flying field. I ran the engine at home on the bench and it was the same way. I changed back to a APC 13 X 6 and the problem went away.

But you have already been there and tried different props.

I have seen some fuels make an engine more rich in the mid range but you have tried that too