engine just goes off: Ignition coil ?

Hello guys, my Healey 3000 BJ8 Bj.66 spins a little. The problem is I drive such a scarce hour completely without props and then at once the speed goes down and it just goes off. After an hour he jumps back on, but goes out again. After 2h you can drive a piece, but then goes out again. Other tomorrow it goes back about 45 min, then end. Have already long a lumenition in it. Yesterday built in a new high-performance ignition coil from Lucas,because the old became so hot, but the same problem. At the ignition coil only nearly7 volts arrive, is that normal? Or lies the problem elsewhere. Would be grateful for any help or idea. Gruss Dany