can only ride with my back binding loose - why?

Here's the story: I can only be comfortably riding my board if I leave
the back leg binding really loose - such that I can move the boot
in the binding a little bit (well, not a little actually). If I don't, I'll just
be falling all over the place and my back leg muscles will become
so sore after only 10 minutest of riding I won't be able to stand up
at all.

Because of my lack of experience and especially lack of experience
with different gear, I can't even figure out what to adjust to get it
right. Tried all kinds of angles, tried adjusting the back support
angle - no go, and what's more embarrasing is that all this playing
around with my gear didn't get me closer to understanging why
is this happening...

I understand that it's hard to figure out what's really going on, but
what would you recommend me to try? I haven't touched adjusting
the width of my stance yet or moving the bindings off the center
axis, but given the fact that I can ride ok with my existing settings
with loosened back binding I just didn't think it could help.

I'm heading for the mountain tomorrow (been on Baker yesterday,
will try Crystal tomorrow), so if somebody can throw out some ideas
I'll try them out and report back.

Or maybe I need a binding that has adjustable tilt?

TIA!

Background:
intermediate snowboarder (1 season)
mostly freeride but will jump anything that I can find on my way down
Palmer Carbon Circle 158 (a bitch to ride for a skinny guy like me, but
I still love it!)
Salomon SP-4 binding
Salomon Synapse boots.
5'11 tall, around 155 pounds (yeah, skinny)