I ran helicomatic freewheels for years. I do not think the issue is they were bad, I think when compared to some other offerings of the time (Shimano/Suntour), they simply were not as good (they had the same teeth design as Regina's and other high dollar European freewheels, just not the pinache').

From your description of the problem, I suspect the problem may be derailleur. Which one are you using and what sized cogs are you tring to shift into?