50 hp merc rpms cut down sometimes

[steplift20]

i have a 1984 50 hp mercury o.b. it runs great but sometimes the rpms cut down like im loosing cylinders and power then it will jump up again and run perfect
i took it to a machanic and after over 400 dollars he replaced the fuel tank and said it wasnt venting right
i only have one switch box but was wondering if it could cause half the cylinders to cut out sometimes?
let me know what you think