longboard is wobbly why?

A friend of mine has this giant longboard dims = 9'6x23x3 he was complaining that longboard is wobbly anyway me and my borther tandem surfed the thing which was pretty amusing have the vid somewhere will post later. But anyway i descided to take it for a spin solo as my buddy was complaining about the board now im not a longboarder but found the board to be crazy wobbly now im a total lightweight at 140lb and 6ft there just doesnt seem to be any reason why the board is soooo unstable. it paddles real good but after take off the board feels like a jackhammer. Im assuming it has something to do with the fins. Its a tri fin with two small sidebites and a larger center. I messed around with the center fin in different configs but to no avail.
Anythoughts? maybe a bigger center fin or bigger sidebites i dunno as longboarding aint my thing but if im having real trouble riding the dam thing i think my mate who is basically a beginer is not gonna have much luck either and he has complained about the same things.
Anythoughts? maybe a bigger center fin or bigger sidebites i dunno as longboarding aint my thing but if im having real trouble riding the dam thing i think my mate who is basically a beginer is not gonna have much luck either and he has complained about the same things.