5300?
You say it won't go over 5300. What were you getting before that? According to the specs, you shouldn't get anymore than 5500 to 6k....
As for reasons.....could be resistance in your plug wires or gap. It can also be caused from the oil injection system running heavy on oil injection. More oil, less burn at higher rpms. Plugs won't fire hot enough.
What all have you done? I think I read a thread you posted on the throttle linkage. This I think is true. Don't adjust linkages unless they come loose. Ninety percent of the time, a problem is not caused by a linkage problem but can be made worse if adjustments are made.
This is an EFI engine. If you were to remove the air induction at the point you can see the throttle plate, set your throttle to wide open. Then, look to see that the plate is directly horizontal to the air flow. If so, it's not a linkage problem. You may have a dirty sensor or even a faulty rpm gage. Does it seem like it's slower in speed now? or is it slower because you see a drop in the rpm gage? How smooth is it through all throttle ranges and how well does the throttle respond?