very common problem on the sapphs,its caused by a stray signal getting in to the rev counter wire (the green one at the coil) and is know to be caused by bad earths in the loom and possibly the ignition amp and the 3 block connectors on the bulkhead,
best thing to do is remove and clean all the earths especially the one on the inlet manifold plenum,make sure the 2 halves of all the block connectors are pushed tight together,disconnect and reconect the ignition amp connector a couple of times to get a good connection,and check all connections on the coil for fraying and loosness