1996 Pontiac Grand Am Shakes when I drive at 80 km/hr or mo

Hello.

I have a Pontiac Grand AM SE V6, 3.1L
A couple of months ago, I had shakes when driving the car, and a mechanic checked it, the code said "cylinder misfire detected in cylinder #3", so I bought a new ignition coil, and the problem was solved.

A couple of days after that, the problem came back. I checked and changed the wire from ignition coil to the spark, because wasn't making a good connection at the coil end [that was what originated the problem. I put an used wire only to test, it worked OK, so I keep using it]

The problem was solved for about 2 months, until yesterday. Now, when I drive from 80 km/hr to 100 km/hr, the car shakes again, like if a ignition coil were having problems. I checked the coil I changed last time, and it seems to be ok, [the pins looks a little burned, but was that way when I changed the wire].

Other thing I dont understand, and could be a clue is: Why the problem starts at 80 km/hr? And IT SEEMS stop at 100 km/hr?

Thanks in advance for your help.