Ok well I have a 2001 Eclipse spyder gs with a code of po300 I've replaced all the spark plugs & wires as well as the fuel pressure regulator and ecm and mass air flow censor and checked the mounts

Is it actually missing or just the check engine light is on?

http://www.obd-codes.com/p0300

P0300 is a random misfire. Meaning all cylinders are misfiring, Its something in common. 1. vacuum leak can cause this. you may have a high idle with a vac leak. 2. not sure on ignition setup on those cars, but if a single ignition coil is used this can be suspect as well as distributor cap and rotor. ( ignition misfire will present itself more so under engine load.) 3. A weak fuel pump, not common but i have seen it. (check fuel pressure should be around 50-60 psi) a clogged fuel filter is possible, but it would choke and stall while driving. check these things first. hope it helps. Pete

If it's just the cel then I would try unhooking battery for at least ten minutes. This will clear the code. Drive car and see if cel comes back on.

If it has a miss you can feel look at your coils. They tend to leave a carbon trace mark where they are arcing out. Also try some good fuel treatment. I use sea foam or CTC. Bad gas can cause a 300 code.

mine's had the same code p0300 turned out to be 2 bad fuel injector's total cost $935