I would be willing to bet that the OP was using a regular poncho (which I believe from his description of the issues he had) instead of a bike specific rain cape.
With the bike specific rain cape he wouldn't have had the flapping issues he described. He still would have had wind issues, just not so much flapping.

My go to rain gear choices is my J & G rain jacket and my RainMates, for all but torrential downpours, then I swap out the RainMates for full coverage rain pants and my rain cape over the J & G jacket. But it has to be a real heavy rain for me to go to that level.