Whats wrong here? Why does my 2003 Eurovan surge and shudder?

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Even after replacing plugs and ignitions coils,the scanner CEL diagnostics code still reads 'random misfire detected on P0300 or P0305'. It then surges, hesitates and shudders when in motion after a stop like after a stop light or in stop n go traffic. My trusted mechanic says it requires replacement of a leaky gasket causing oil to enter the chambers and costs $800. Any advice is welcome.