JVC camcorder

I think I posted in the wrong section yesterday. Anyway, I put a JVC GZ 300 camcorder on top of the scope on my FX 500. Worked fine for a short time. Then it quit focusing past about 50 yards when zoomed past 30X. Focused fine close up zoomed. I bought another one just like it so I could use the same battery and software and put it on the scope. I checked it at 100 yards zoomed to max and it worked fine. Very clear. Then after about two weeks of filming it did the same thing. It is blury past about 50 yards when zoomed past 30 or 40X. This seems wierd that two camcorders would mess up. I have tried reseting to factory settings and read the manual but nothing works. JVC said mail them back but I don't want to. No one at any camera or video store knows anything it seems. Can any of you guys help? You are the best resource I can think of. Thinks, Scott