You'll have to make up your mind what the problem is. If the springs are too soft, you probably just need new springs to original spec. Your springs are probably just old and knackered. Putting heavier than standard springs in, without having tried good standard ones, could be a mistake.
If your problem is insufficient damping, there may not be much you can do, apart from trying thicker oil. SAE50 engine oil is available and is very thick.