Nullspace(M) takes a lot of time for a big matrix

Hello.
Thanks for the answer on previous question. That helped a lot.

Now i'm trying to find a nullspace of a 243x270 matrix. It has a lot
of zeros. Actually, it has nonzeros in 5 place in each row. Some of
nonzero elements are 1, -1. Some are symbolic a, b, c. ( a, b, c are
declared constants. )

The problem is a that calculating takes a lot of time, while nullspace
for the same matrix in Matlab is calculated ( symbolically ) for like
several seconds. In Maxima it takes > 15 minutes and i didn't have
enough courage to wait for finish.

Is there any algorithm or workaround to fasten the process of finding nullspace?

Thanks in advance, ptomaine//