I cannot comment on the specific problem of the driveability concern...but when you have a blinking check engine light it usually means its running rich or bad misfire. And its in the process of causing severe catalytic converter damage. So it needs to be addressed immediately.

 

bkey71 right on with the flashing CEL. If the CEL is flashing it indicates a potential catalyst damaging missfire. The p0421 is often right behind as this is a CAT efficiency code. You should also have some kind of missfire code being stored along with the P0421 such as a P0300,P0301,P0302,P0303,P0304, P0305,P0306. Are any of these codes also being set? These are missfire codes and if the light is blinking at least one should have been stored as well. Would look first to the plug wires and spark plugs as potential source of the problem as is fairly common, especially on the 4 cyl engine given your description.

 

You were right on the money. I pulled those plug wires out from the spark plugs and found out they had collected some fine dust/dirt. I found that two of the four plug wires had small discolored traces running down to the head on the inside of the boot. I cleaned them up and my engine is running like new again. Thanks again for all of your help.