check my demo in the code base, i use something startlingly similar for jumping...
http://darkbasic.thegamecreators.com/?m=codebase_view&i=456fc68cb31ab4cbd476030a4f6e52a9
EDIT: What? is that mine?!? it even uses the same variable names!
here's mine
if jumpb=1
height=height-1
position object 4,object position x(4),object position y(4)+height,object position z(4)
endif
if object position y(4)>100 then ajump=0 else ajump=1
if object position y(4)<100
position object 4,object position x(4),100,object position z(4)
jump=0
height=0
endif