I second Jeff's recommendation for Sun Tour "retrofriction" downtube levers but the clamp-on version he referenced isn't the only one. Sun Tour offered these levers in braze-on type for several of their road groups and the "Cyclone" version should be available at modest cost.
Also, any Shimano indexed downtube lever set will work since they all have a friction option. Six and 7-speed Shimano lever sets are obsolete so many bike shops will be happy to sell atheir NOS stock cheaply.
Either Sun Tour or Shimano levers will solve your "ghost shifting" problems.