Since this is your first post....
Welcome to the M31 forum...
From your description it sounds like oxidation. The PC is a great tool but it's never going to be as powerful or as effective at removing oxidation as a rotary buffer on a wool pad.
I'd recommend getting a good wool cutting pad and re-cut your hull using the rotary buffer and either an aggressive compound or a medium cut compound.
After that you can go over the hull with either your Porter Cable and a one-step cleaner/wax or a wool finishing pad on your rotary and a one-step finishing wax.
The above is a recipe for a two step approach which is a lot a work for a boat this size but there's only one other way to cut aggressively and leave the hull protected without leaving a swirled out mess on a dark gel coat and that's to use a Flex 3401 with a one-step cleaner wax.
The Flex 3401 is a forced rotation dual action polisher. It works really well for removing oxidation and laying down a coat of wax at the same time and you can use it with both wool pads and foam pads.
Here's a link to the Flex 3401...
http://www.marine31.com/best-boat-buffer.html
You can try the tools you have try to do either a one-step or two-step to your boat, really comes down to you and how much time you want to put into the project.
I think you'd find the Flex a great way to maintain your boat and keep the process to one step.
Won't leave the hull looking scoured or swirled like a rotary but has a ton more power than a Porter Cable "style" DA Polisher since it's gear driven like a rotary buffer.
Here's a video that shows the difference in the tools...
http://www.youtube.com/watch?v=Arn_kVH3BVc
