buick LaSabre Limited 2000 is vibrating with you speed up and not sounding right

I drove my car on saturday and it was driving just fine but when I got in my
car on sunday morning to go to work It started just fine and i let my car warm
for about 5 to 10 minutes.  Then I put it in gear and started to feel a vibration
that I had not felt before.  I drove the care less than a 1/4 of mile and turn
around and went back and parked the car.  I had a friend look at it the next
day when it cooled off and All I was told was they thought It was my coil
pack.. my spark plugs.. or my spark plug wires.. lastly it could be my my
injectors. I have now changed everything but I have not done anything to my
injectors since my gas tank was almost full when it started acting up. I am at
a loss with limited knowledge of cars and funds. The check engine light
keeps flashing when the car is on and when I have test driven the car after
each thing was repaired it feels like the car is running in 5 instead of 6
cyclinders.  maybe i am just overthinking things.  i know my car is not right.  
the code i was told that my car is showing is mutilple misfires. and that was
after the spark plugs were changed but before the wires and coil packs were
replaced.