Weird problem

does this ever happen when you start the car in the morning? it almost sounds like its struggling to get air. Since the car has 91K on it, I'd start taking care of all general maintenence items first to see if that helps since its not too expensive and your car probably needs them anyways.

Is the car throwing any codes? if the cats were bad, his check engine light would be on.

How old is the airfilter? Has the IAC been replaced or cleaned? When were the plugs last replaced?
 
Sounds clogged cats. Being you are in Cali sucks because you can't just put on an off road pipe and go on your way. Try the plugs and see what happens but I'm betting you will have to replace the cats. Try classifieds to find someone selling a stock H. A lot cheaper than paying a shop to replace.

Plugs are fairly easy to change. Make sure the engine is cold when you do. Likely you don't have a torque wrench if you are new to auto maintenance but getting them tight then making an extra turn on the socket wrench should suffice. After a few hundred miles it would pay to give them another whirl with the wrench to make sure. The gap is listed on the hood sticker in case you forget or want reference. The right tools are a plus. Should have a few extensions and a swivel...3/8 drive socket wrench would be fine. Most Sears you can buy a pretty decent set for about 50-75 bucks.
 
I am not sure but I think due to federal regulations the warranty on emissions controls like cats is really long. I think it is 100,000 miles and I think if you take it in you may get a warranty replacement. Call the Ford service counter...

Be sure and update the thread when you find out what the problem was and tell us how you fixed it please.
 
Have you seafoam'd it yet? Might as well clean the MAF too.

Are you throwing any codes or is you CEL on?

Other ideas are the EGR, IAC, plugged injectors. bad coil pack, plugs fouled or gapped wrong.

Did you change or add anything before this started to happen?

~K