RE: kombat 97 fin sizes
When powered up on your 5.8 with the 280 fin, is the board tending to spin out - if not I would not go bigger. Have you tried the fin in different positions in the box.
Are you in waves or freeriding straps inboard or out?
With the 6.2 Saber I would suggest outboard straps move the fin back.
It may just be technique in lighter winds, have a look at this it may help
I prefer to sail overpowered so I've not run into a simalar problem with my K95.
You have plenty of variables with setup on your board/rig before needing to try another fin.