How to fix a Toshiba DMR E55 DVD recorder with self checking error?

I have 2 of these recorders showing the "self check" message on the display. I have purchased a repair kit for this fault which replaced IC001, D007, ZD002 ZD004 and ZD107 and the same problem is there, what else can I check / replace?

It will play pre-recorded commercial DVD's fine but put any blank or recordable disc in and it try to read it and then display the message with the occassional reboot. The type of disc makes no differance, DVD-R, DVD-RW or DVD RAM, all cause the self check message to appear but any pre-recorded (not home made ones) disc plays fine.

Any ideas?