p0300 millenia misfire

I have a 2007 Mazda3 2.0L w/ 71,000 miles and have been experiencing a rough idle. Eventually the check engine light came on. I had a mechanic run a diagnostic test and code P0300 came back (random misfire). Upon further inspection, he recommended I replace these items: Crank position sensor 4...

I have a 2002 Mazda Millenia, 2.5 l. I am getting engine rpm fluctuations of 500-600 rpm, even after popping transmission into neutral and trying to hold rpm at ~ 1700. At other times, I get a very short, single hard jerking motion. Occasionally, the sets the multiple misfire trouble code. Reset...