Noise when using Modulator & Catv

I'm trying to modulate my camera and a vcr using a three channel modulator
from
http://www.partsexpress.com/pe/showdetl.cfm?&User_ID=16133629&St=3293&St2=49720789&St3=81068843&DS_ID=3&Product_ID=4369&DID=7
I'm having problems though.  I have everything hooked up about 4 feet into
the house and then combined and then sent to the remaining of the house.  If
I hook the modulator up without using the combiner and just send it
throughout the house it looks great and clear.  The cable is the same, looks
good by itself.  The minute I hook up the combiner everything goes to hell.
The modulated channel is super fuzzy(ch 83) but the cable looks decent.
I've tried a booster on the output of the modulator to the combiner and
really didn't help and also tried the booster after the combiner with the
same results.  I'm using a radioshack combiner/splitter and have tried a
Magnavox splitter also.  The problem seems to be in the combiner but don't
know how to fix it.  Any suggestions?  I thought the modulator wouldn't have
this problem?

Thanks,

Barclay