I'm in the same boat big fella. Deadlifts are just a damn finicky lift for some people, me included.
FWIW, I think you are not letting your programs work for you and are winging it too much. On the Cube you should never fail a lift, so my suspicion is that you over-reached with those singles assuming well x amount of time went by so now I should be able to do that amount of weight. I don't think it works like that especially for deadlifts.
Definitly get your form fixed up, but as important in my opinion is patience. My deadlift has barely gone 20 lbs in a year, so I know patience is a bitter pill to swallow, but I keep telling myself I will get it one day and I'm following a plan to get there.
You can do it. It will just take some more time.
I am Anton Zdravko Martin!
Best meet lifts: Sq 150 Kg (330 lb), Bench 120 Kg (264), DL 160 (352) @89 Kg (197)
Best gym lifts: Sq 375, Bench 280 (pause), DL 385 @205 or less
Goals beyond my wildest dreams, 600 Kg raw total, 200/160/240
Hack away at anything which isn't essential. Do what you love, and do it often. Fazc.
Everything competes for recovery so more assistance is not always the best idea. miked96