This is a very common problem with the pioneer ipod cables. they are crap. if you search on car audio forums its the same case over and over. what i reccommend and have had no problems with is using the apple brand cable that is the same thing as the pioneer cablebut alot better quality. its 35 bucks at best buy or 50 at the apple store. heres a link. hope this helps.
Apple Composite AV Cable - Apple Store (U.S.)
heres a link to the crappy pioneer cable, you can see the difference.
iPOD PIONEER AUDIO & VIDEO CABLE CD-IU230V CDIU230 NEW - eBay (item 250465770195 end time Aug-14-09 22:24:38 PDT)