HELP! Dollar Bill Test? Distributor? Valve?

I have a 00 Mercury Villager (v6) 70,000 miles. It's missing a little, just a slight backfire and a little sputtering when idling (no Check Engine light). It's definately not running perfect, but it's still got good power and you can't even notice this problem when driving it.

I've cleaned the throttle body, new cap, wires, plugs, rotor. No help.

I was told to try the "Dollar Bill" test. Simply hold a dollar bill over the exhaust pipe and see if it gets sucked in. No way right, but it really did!! Every few seconds instead of blowing out the dollar was sucked in! I've tried this on other vechicles and it doesn't happen on them.

The person that told me of this test, said it indicates a distributor or valve problem.

What do you all think, what else could cause this? Is there any way I can narrow the problem down between a valve/distributor? Kind of major stuff with only 70,000 miles! Could a clogged fuel injector cause this type of problem (sucking the dollar in like that)?

Thanks, I really appreciate your help!

my guess would be timing belt