Hi Peike!!
I had lots of trouble with positioning in the bassinette and crib as well. What we found worked well was to elevate the crib slightly using towels under the mattress, or books under one end of the crib legs. They are "sliders" though, aren't they!!
They make a few products these days (check Babies R Us if there is one near you) that are slightly better for holding your lo in place. (have heard good things about this sling -
http://www.tuckersling.com/)
But if it's gas, there are several things you can do to help.
"Bicycling" the legs, gentle tummy massages, and simethicone drops seemed to work well at our house. If you can, try out different bottles that either "vent" the air, or something along the lines of Playtex Nurser's, which are supposed to help amount of air that gets into belly.
There IS light at the end of the tunnel though - after their "4th Trimester", when they are 3 months old, their digestive systems seem to straighten out quite a bit, and passing gas tends to be much easier on them!!
Hope I've helped a bit!