Random Misfire Error Code & Catalytic Converter Gets Hot

The Problem
The current problem with the car is that it is not accelerating fast, for example i would
push the petal to the metal and only go 25 mph  and my RPM would be 4000. Initial
my brother and cousin though it was the Catalytic converter because it was getting hot
but that has recently been change along with the fuel filter and spark plugs. but the
catalytic converter is still gets red hot so i forbid myself from driving it till i fix what
every causing the slow acceleration and the temperature of the Cata when i took the
error code think P0300 random misfire to the local Autozone they printed me a long list
of things that could be a issue my question is what is the likely culprit so i can fix
those 1st before shelling out cash to rebuild a engine. may just have to get rid of it at
that point. so i guess what am asking is a list maybe top 10 so i can fix them one by
one as my money comes in.

Before the problem
Before the accelerating issue the car would randomly stop when i slowed down or
stooped that when i replace the spark plugs.   

Any information will be helpful even if you just point me to another forum that has a
solution, Thank you in advance for the help.