Down tube shifter/indexing help needed

Hi

Fist time posting on this forum but I am in need of some help so here goes.
I have recently purchased a Condor Super Acciaio frame and have built it up using some bits from another bike but mostly new parts. I have fitted some Dura Ace SL7900 down tube shifters, paired to a Ultegra 6800 rear mech. The down tube shifters and rear gog are 10 speed. All bits mentioned and cables are brand new.
I have tried for two hours to set the gear but without success.
I line up the rear mech with the smallest cog, attach the cable to the mech, then start to go up the gears using the shifter. This is ok for 3-4 changes before I start to get some chain rub on the gog, this is normal and disappears with a few turns of the cable barrel adjuster. It then shifts up through the rest of the gears fine. I then shift back down but when I get back to the bottom three gears I get the chain rub again. Problem is it only goes by taking off the turns of the barrel adjuster I haad just added which means I am back to square one!!!!
I have tried tightening the cable, I have tried loosening the cable but I cant get it to run how it should.
Do I need to get a dura ace rear mech?

Please dont suggest STI shifters, I want to run downtube shifters, up front I have Campag Record brake levers which look awesome wont change them. I just need to get the indexing running properly.
The shifters are indexing by the way, the front mech is friction.

Thanks in advance.

Lee

Had something similar with an ultegra rear mech. After many hours indexing, re-indexing changing cables etc .etc. it turned out to be a weak return spring in the derailleur.

Is the mech hanger straight?

Shouldnt be a weak spring in the mech as its brand new, can you buy replacement springs?

Hanger looks straight with no dents or dings.

The derailleur I had a problem with was on a brand new Bianchi hence initially discounting a mech problem. Changed the whole derailleur in the end, see if you can borrow one to try.

Thanks very much freezing77, I will swap one off my other bike and see if that cures the problem.