If your idle was at 700-750 , you probably did not have a vac leak, until you took it apart.
The flashing CEL means you are about to destroy one or both cat converters.(do not operate the engine except for testing purposes , shut 'er down immediately if you still get the flashing CEL) Probably from unburned fuel getting into your exhaust due to the mis-fire, or from an excessively rich mix. It could be that the lean condition caused by the vac leak is causing the 'puter to pour in more fuel.
If you no longer get the 304 code , then you may be on your way to happiness after you replace that leaking gasket.
2003 Intrepid SXT 51,000 miles 1-10-06
'94 3.3 Intrepid owned since May '94 182,000 miles has been traded in for the above.