hmm... most stumblers haven't had too many problems with the mag mounts on them....
you may have a bum antenna...check to make sure your connections are tight also... you might want to check with Radio Shack to see if they have an SWR meter that goes up -> 2.4GHz - this will test out your antenna to see how well it is performing....
You might also want to check out
www.aesham.com a ham radio site with a bunch of antennas and accessories...just an idea
search the threads for more info on the antenna...