If your clutch is hooking up good and you are maintaining 3100 rpm’s on the engine, I would look for the blades to be out of alignment with each other, meaning the deck floor may have some warpage. You need to get on a level surface and with the deck down, tool bar chains loose, get under the deck and measure the actual blade tip distance from the floor in four positions, 12, 3, 6, 9 o'clock on all blades. This is the only way to know at what height each blade is cutting. Even though it's a old machine, when it was new it cut pretty dang good. Something other than the blade configuration is giving you the issues.