Engine won't tickover in gear

Hi all, I have a 2000 sc of 72 vintage, genuine 80 thou miles, stood since 95, then I acquired done loads of work, new pistons, rings, carb, all the usual ignition stuff, head gasket etc, engine runs really well, responsive, lift the carb pin revs rise slightly then drop, engine revs like a racing car, tickover is high compared to the v8 auto I had, but as soon as I put in d or r the ignition light starts flickering and just stops running after about 30 seconds, I'm flumaxed, I just can't put my finger on the problem, tried riching it up but then runs lumpy, checked the timing and that seems fine, any ideas what to try