Electrics die at 3000 revs

Hi all,
I've just changed the gearbox and now when I get to 3000 revs the speedo drops to 0 and the LCD display dies. The lights still work and the engine doesn't die
When I drop the revs or change gear it settles and comes back on.

Is there an electrical connection I've missed? Bat-gearbox earth is clean and tight, I tightened the +ve terminal too (it had gone loose)

Any ideas?