I've got a 25 ft cable that goes between my source (dvd player or HD tuner) and my projector, an older Hitachi HD, with no sign of weakness at all. My receiver is a Denon 3803, with only component video switching. I have no sparkles, but I spent around $200 on the cable.
When I tried an HDMI switch box, I had hand shake problems, but that may be because both the tuner and the projector are DVI, only the DVD player is HDMI. The cable is HDMI with DVI adapters where needed.
Anyway, I wonder if your unaccessable cable is the problem. But maybe Onkyo could give you advice on something that works with their receivers.