1997 Buick Lesabre

I would look at the p1406 first that code can cause the code P0300 if the egr valve is sticking open it can cause a vacuum leak causing the P0300 random miss fire code.I would start by removing the egr valve turn it upside down take a small screw driver you will see the spring loaded pintle start pushing it down see if it's stuck at first when pushing it down. Then slowly bottom it out then slowly release it and see if it sticks closed or open.

Was it sticking open or closed?

If you checked it like I told you and found it sticking opening then I would replace the valve and clear the codes and see if any codes come back.