Interface problems TC + lens

Has anyone had interface problems with the Kenko 1.5x TC and Sigma lenses?

My TC works sometimes with my 70-200 but often the aperture reads 00 and the lens effectively becomes manual and this weekend i even got my first Err 01. The TC is fine with my 24-70 though and works 100% of the time. Without the TC the 70-200 works perfectly 100% of the time.

2470+TC = fine
70200 = fine
70200+TC = intermittant problems

From this i can deduce that the lens and TC are both working just not with each other

I've cleaned the contacts on the TC and 70-200 with no luck, tried twisting the lens on the mount so see if there's a bad contact.

The only thing that i can think of is there is some problem with the 70200 and TC interface. Anyone have experience of this?

Ok, as an update to this it seems like a problem with the 350D using a t-con on a sigma lens, my 5D works perfectly with the 70-200 and t-con.