Stutter while maintaining speed grand marquis

I have a 2004 mercury grand marquis just bought it it
drove fine for a day I notticed what felt like a small
misfire so when I accelerate it’s fine but maintaining a
speed I have an issue codes that came up were the
two catalyst codes I think po300 po400 something
like that then I bought the plugs changed them now I
have po316 misfire upon start up first 1000 rpm or
something then I have a cylinder 5 misfire so far
what’s been done to it is alternator before me water
pump before me looks like new ac lines before me I
did sway bar link and plugs I just ordered the coils but
I don’t want to dump a bunch of money and hope to
solve it I’m trying to figure out what’s wrong with it I
was thinking injectors so I swapped cyl 5 out gonna
see if it jumps cylinders any ideas of what it could be
and I really need this car to last til I can save up
money to get what I really want  any questions let me
know