It sounds like the cutter clutch is slipping; this may be because the cutters are out of adjustment or the clutch plates and springs may be worn.
You are obviously attending to the on-cut adjustment, so if this does not cure it then you will have to remove the cutter clutch assembly. To do this you have to remove the four 3/8 UNF bolts holding the bracket onto the side frame. Take off the belts as well, and then the whole assembly should pull off the shaft. I say “should”, but quite often an older one would be seized onto the drive shaft so you have to get a large pry bar and use a bit of excessive force.
Once you have the assembly removed the only way to start dismantling is to put the end of the tapered key (holding the pulley on) in a vice and smack the end of the centre shaft with a brass drift to separate them. You should then be able to take the pulley off the shaft; the next part is to release the locking ring on the bearing and remove the bracket from the shaft. The clutch can then be slid off the shaft.
To dismantle the clutch assembly there used to be a special tool which compressed the three clutch pins so that the snap rings can be removed, then when the tension is released the clutch comes apart easily.
We always used to replace the pins, springs and snap rings when servicing the clutch. The “ears” on the outer plates used to burr over, and we always used to file these burrs off before re-assembly.
That should cure your problem. If you need parts I understand Bartrums Mowers bought all of the old stock from Ransomes when they got rid of their obsolete Motor Triple spares.
Good luck.