2002 Toyota Corolla PO300/PO302 codes

I am still trying to diagnose the above codes, and one of the possible issues is a bent valve. This would be a rather expensive fix, but if that's the issue wouldn't the car be idling rough at all times and lack power when driving at all times? The car idles rough for about 1-2 minutes only then seems to run very smooth and no lack of power upon acceleration. I have the same problem with a dead cylinder, would it lack power at all times if this was the case? I am waiting for it to warm up outside so I can stand being outside to perform a compression test. What other possible issues could cause this-I know there are numerous!

Thanks ahead of time for info-VERY helpful website