I think I would first change the oil to a recommended oil- I use rotella
as to your inconsistant power issue, may be a tank venting issue. Try properly mounting your gas cap and making sure the vent hoses are not restricted.
If after changing oil still slips- try going for aftermarket springs- OEM prings are known to be weak.
Check your fork alignment, your front may be fighting the rear and showing up during braking. Check your steering bearings for slop - and make sure all wheel spacers are present. <-- some guesses.
All this stuff is pretty easy, so good news.
