A new freewheel costs 20 bucks or less. Unless you are on a very tight budget, this might be the easiest way to go. Then the new chain and freewheel can wear together and your problem is solved. I understand wanting to reuse anything you can to save some bucks, but if you're not happy with the performance and are spending a lot of time swapping cassettes without significant improvement, it can be false economy. If your garage sale hybrid is a temporary solution that will soon become your crappy weather, Plan B bike, I'd just put the original freewheel back on and live with it saving the money to go toward the new Plan A.