if this is his ford 850 from his profile, then there could be a few reasons.. but most will circle around his cam follower pin ( and or cam ) being worn down and needing repalced ( pin ) and built up ( cam ).. plus it will then need a linkage adjustment using the alternate method NOT using the nuday tools as outlined in the service manual.
I bet manipulating the draft lever makes the lift go to different heights too.
as for the bobbing.. that is usually leakdown and circuit regen thrut he unload valve. leaks are usually at piston seal to cyl.. though could be in cyl to top cover.. at safety/relief/check valves too.
I&T fo-20 manual will cover this repair fully...
soundguy