Need Opinions/advice: Changing the shaft of Maruman Exim Nano driver

hello all,

an equipment newbie here.

all my clubs are in factory settings.  i have never changed anything in my clubs.

so, i have no knowledge about club fittings, changing shafts, changing grips, etc.

my father received maruman exim nano driver as a gift a while ago but he does not use it as he prefers stiff shafts.

here is some information about the driver: http://www.golflink.com/golf-equipment/product-detail.aspx?p=326535

as far as i know, this is a non-conforming japanese version that is no longer being produced because of the spring effect COR value over .830 and hits the balls too long.

my father told me he had no plans to use this driver and i could do whatever i wanted with it.

i took it to the range and hit some balls along with my friends.  we all agreed that this driver hit longer (at least 20 yards longer on average among 5 of us) than any of our drivers (taylormade r9 460 9.5 stock aldila stiff shaft, cobra xspeed, taylormade burner 9.5 stiff shaft, taylormade r11 stiff shaft).

however, we also agreed that the balls were traveling left and right, pretty much everywhere.  i could actually feel the shaft bending/flexing when i swung the driver.

i think it was because all our swing speeds were over 100 (mine clocks at 118), and the regular flex shaft was just too flexible.  it's just my guess that i pulled out of my butt.  please correct me if i am mistaken.

what i want to try is to find a decent stiff shaft and replace the graphite shaft on the maruman driver.

before i take the driver to a shop, i wanted to ask the sandtrap members if this would be a good idea.

if it is not, any explanation on why?

if it is, any recommendation on the shafts? something that wont break my break if possible. =)

thank you in advance.