2011 traverse misfiring

My car was just about to hit 100,000 miles and the
check engine light went on when I was driving up
an incline on the highway and started to hesitate
like it was misfiring. Took it to my mechanic we’re
he said it was the spark plugs. Had them all
replaced and he flushed the fluids(said there was a
lot of gunk).  Next day it happened again. Took it
back, said it was a cylinder but wanted me to bring
it to a dealer. The dealer told me it was cylinder 6. I
had my mechanic  fix it since he started with it. I
got it back and it drove smooth for my 10 minute
drive. I took it on the highway to go to work and
once I accelerated up a hill it started to misfire
again and the check engine began to flash. I
slowed down and it the light went off. I stayed at a
steady speed but when I would accelerate at all it
would misfire. Is there another issue besides the
cylinders? It’s been going on for 3 weeks now and
I’m getting tired of it.