P0300 Error Code and rough idle.
 

Crazy laptop keyboard
 

So, I typed this in Microsoft Word and pasted it. It was perfect in Word, but here, it skips spaces etc. Hope you can make sense of it.

The O2 sensor may have been damaged while left hanging there. A bad O2 sensor can give you a multiple misfire code. It's worth checking it anyway...

Can it be the Catalytic Converter????
 

CWBDDSPC I really was not driving it with the O2 sensor hanging,
just setting in the garage. However when I remounted it, the P0300
reappeared. Now, I tried removed all 3 or the O2 sensors and left
them plugged in but just hanging. The engine runs smoother and
when I accelerate to 2000rpm, everything gets even better. The check
engine light does not come on now, but the code reader says I have
a P0300 Random Engine Misfire "PENDING" message. There never
has been an O2 failure message. Still wondering if the catalytic
converter is the problem. No error message to indicate that. Any
cheap way to determine? Somewhere, I read that takes expensive
equipment.

If the O2 sensors were simply hanging from their wires, they may be damaged from simply hanging there. I realize they are not THAT heavy, but the little wires are not designed to support the O2 sensors either...

Also, perhaps damage was done to the wires or sensors while removing or installing them

Do you have reason to suspect you have a problem with the catalytic converter? From your original post, everything seemed fine until you needed a head gasket correct? Nice job diagnosing that! Then everything was good until you got the multiple misfire code. You mentioned recently working on a $30 sensor that could be a cause the 0300 code. It seems logical to me to rule out the O2 sensors before blaming the cat. You can test the O2 sensors with a DMM and a heat source. If you are able to rule those out, great job again! Then there are a ton of posts on the forum about the P0300 code. There is one in particular that leads you down the troubleshooting path quite well. I would look for that and start ruling things out. Yes, a bad catalytic converter CAN be a cause, but it wouldn't be my first guess in your situation. And guessing gets expensive...