I know I am fairly new here but the fact that I only have 10 post does not show how much I have actually read this forum. I have literally spent at least 20-30 hours reading here and in almost every case found the info I was hunting. I guess a good thing about having a 12 year old car is that chances are good someone has had your problem in the past. In any case...

I ended up with these codes about 500 miles after I replaced my engine with another used one. I had a spun rod bearing in the original. Being that soon after the replacement I was sure I had done something wrong. A LOT of reading and talking to a few mechanics convinced me this was coil related.

Well since I had an entire second coil pack including ignition control module from the junkyard engine I just swapped the entire thing. My problem did not get any better.

I then took an extra spark plug and plug wire I had wrapped a ground wire around the plug and hooked it up one at a time to each coil and started the engine. Every coil would make the plug spark.

I did notice however that one time I left the plug out of the plug wire and a spark jumped almost 2 inched to a ground so I decided to try another test. I would take a plug wire hooked to a coil and just laying on the plastic fan shroud and then take a ground wire and see how close it had to be to get the spark to jump. This is how I found the problem. On two of the coils a spark would jump a 1 to 1.5 inch gap to ground. On the third coil it would only jump about a 1/4 inch gap. So the coil was working but weak.

I swapped that one coil and the car now runs fine and I am not getting any codes.

The reason I had such an issue was I had one weak coil in both of the two coil packs I had.

I should also add that in addition to the codes the symptoms off my problem were the car would shake and vibrate under light acceleration and backfire going up a hill. You could cruise down a flat stretch of highway and things would seem pretty good. You could also nail it and while it would hesitate to get there it would run fairly strong when it got to 4-5000 rmps.

Maybe this will help someone with a similar issue

 

well the 300 code is a random cylinder misfire which is usually something other than the coils and the 305 is a misfire on cylinder 5 did you put new plugs and wires in when you swapped the engine... if not your going to regret that these cars "designed" to never change the plugs meaning its a royal PITA

actually these cars were "designed" to never be worked on... hey GM maybe a little room here lol

 

I did put new plugs and plug wires in when I swapped the engine which is one of the reasons this threw me for such a loop.

 

well at least you got it fixed and save a camaro from the crusher

 

Something is probably wrong with your valve train. You never know with used engines. I would take it to a good shop and have them diagnose it for you. Thats what I did with my Camaro and it turned out that I had a broken valve spring. My car's symptoms were very similar to your car's.