I don't have any advice (yet) on what is wrong, but I thought I'd reassure you this works. I have 5 Associated B4 buggies in this exact setup. 3 have the GTB, 2 have the XBR. I use 1 XS3 Pro and 4 XS3 radios. I have run both lipo and 6-cell batteries. I don't have any problems as you describe.
One idea - is the buggy chassis composite or carbon? Might be the antenna touching something conductive? I don't know the losi buggies.