Hi Sami,
It sounds like you have a good amount of experience.
But we don't have a description of the type of miss-register: front to back, side to side, or a cocking of the sheet?
Are you using metal straps at the edges of the sheet to keep the stock from running over the far guide feather spring? And do you move a feed board tape over right near the far guide to keep the stock from going under it?
As for the feed board wheels, my new machine came with one pair that was not the of the ball bearing type, but solid hard bakalite ones that were lighter than the ball bearing ones. Make sure that all feed board wheels, especially the ones at the tail of the sheet are clean, lubricated and free turning.
Incidentally, you have not mentioned if it is a particular stock size that gives more register trouble.
The feed roller pressure at the stop fingers, is set as a timing: something like turning the press forward by hand with no stock fed, while testing with a finger to find the point at which the upper steel feed roller just contacts the lower rubber one, continuing to turn and counting, from that point on, the number of whole and partial turns of the upper feed roller until it is free again (there is a hole on the left side of the upper steel roller to use for counting the turns, set it horizontal just as they make contact). This should be 1 and 1/4 turns of the upper feed roller. Check the manual on this for where and what to adjust to achieve this setting. Now, the up/down movement of the frame that holds the upper feed roller (the elevating frame), is actuated by a cam follower on the side (the left one I believe) which rides a cam bad on the blanket cylinder. It could be that wear due to lack of lubrication has developed to the extent that that little cam follower wobbles and walks off the cam band when things warm up after 100 sheets or so. There may also be some undesirable play of the elevating frame. Get rid of it.
Another, perhaps stronger possibility is that the gripper pads on the cylinder grippers are either not set correctly, and/or the rubber coating on the tips of those grippers is worn and needs replacing. As I recall, there is a separate service manual for that press. Do you have that? Try getting it from Ryobi, a dealer, or on e-bay. It will explain how to set those grippers. The service guy who sold you the press should have set these. Did he claim to have done some basic restorative work on the machine prior to selling it, or was he "letting it go as is" at an attractive price? Anyway, if those are set correctly, but with worn rubber tips, then the solution is to replace them by new/newer ones. As a short term fix, the ones on the very ends on each side may be less worn than the rest and could be moved in toward the center, until new replacements arrive.
Report back.
Al