courtney, here is part of a service bulletin addressing this issue. RANDOM MISFIRE (DTC P0300, P0301, P0302, P0303, OR P0304)
TECHNICAL SERVICE BULLETIN
Reference Number(s): TSB-06-13-004, Date of Issue: April 01, 2006
MITSUBISHI: 2003-2005 Lancer
Superceded Bulletin(s): TSB-05-13-003, Date of Issue: February 01, 2005
PURPOSE
NOTE: This TSB supercedes TSB-05-13-003, issued February, 2005, for new diagnosis and repair information.
If the Check Engine light comes on and DTC P0300 (Random Misfire) or P0301, P0302, P0303, or P0304 is set in the PCM, the cause may be one of the following:
Fuel injection/Ignition system
PCM software
Worn input shaft 5th gear spline, misjudged by the PCM as a misfire
To eliminate this condition, follow the diagnosis and repair procedures described below. These procedures are outlined in flowcharts in ATTACHMENT A (2003-2004 MODELS ONLY) and ATTACHMENT B (2005 MODELS ONLY).
since I do not know if yours is an AT or MT I did not post the part pertaining to the MT. Other issues with this could be a bad ECU or bad injectors.