2005 Mazda6 Comp Code PO300 Engine Running Rough

 - 2005 Mazda - Mazda6 - V6

- Automatic & Manual Transmission

- 111,000 miles

- "Check Engine" light on - Computer Code PO 300 "Random Misfire"

- Replaced spark plugs - seemed to correct problem ("Check Engine" light off, idled smoothly) just for couple of minutes, then "Check Engine" light returned, rough performance, including idling rough, returned immediately

- "Check Engine" light remains on, car runs rough, missing, in both automatic & manual transmissions

- Alot of difficulty going up hill, have to use manual transmission to shift just to make it

- Automatic transmission seems to shift way too quickly through the gears on up to 6th gear before reaching 30 mph

ANY SUGGESTIONS AS TO HOW TO DIAGNOSE & FIX THE PROBLEM??