Jeep Grand Cherokee question

Recent my Crank Sensor went out and I had it fixed. Car ran great until a few
days later I have accelerating up a hill ( we live in West Virginia) and all of a
suddent my check engine light started flashing and the car began to shake.
Got it to the shop and found the 5th cyclinder was misfiring and ended up
replacing the entire coil pack. Left and a few days later went to accelerate up
the same hill and my light starts flashing again so I took it back to the shop to
find that nothing was coming up so we replaced the spark plugs. A few days
later i'm once again driving up the same hill and the light starts flashing again  
but then goes off. Take it to the shop and nothing comes up on the
diagnositic test. Noticed my car was running rich a good majority of time my
mechanic decided to change my catalytic converter. left the shop everything
seemed fine. a few days later i'm driving up a hill and my check engine light
starts flashing again then goes off once I let my foot off the peddle and stop
accelerating. There is no shaking present (that only happened when i
needed my coil pack changed) and the car accelerates just fine. Please help
i'm at a lost and don't know what to do!