When you cleaned the carb did you disassemble it, soak it, and reassemble with a kit?
If so was the accelarator pump bad and a new one installed?
Did you check the fuel tank antisiphon valve sticking?
You should pull the plugs and check for evidence of water intrusion!
Have you run a compression check to see if all cylinders are functioning?
Is the motor fireing on all 8 Cyl.?
All of these can contribute to those symptoms, a small 8 will idle fine on 7 cyl. be hard to plane, and then run again once over the hill to a point. Go back to basics ck spark, compression, wires, choke, etc.