[plt-scheme] isolating memory leaks

My program seems to be creating circular structures, but I can't find 
an an effective way in MzScheme to isolate the problem.  I've tried 
bracketing code with
(begin (collect-garbage) (current-memory-use)), but garbage collector 
doesn't seem to pick up everything on every pass, so repeated tests 
don't return consistent numbers.

is there any way to force the garbage collector to be maximally strict?

Can anyone suggest any other tools or techniques that I might try?

Thanks,

David Goering