My 94 LeSabre sometimes vibrates or shakes when going on the road. It does it mostly when I am cruising at about 50 mph. It's something like an hesitation but somewhat rhythmic. If I lift the foot from the gas pedal or press it until the car shifts down that vibration or hesitation stops. It also does it at lower speeds. It's not the CV joints or the computer. I took it to a transmission place and they said it must be the engine, maybe a bad spark plug, wire, etc. I replaced these and also switched the Ignition Units and Coils with the ones from another Buick I have and it didn't fix the problem. When I read the codes one tells me that cylinder # 5 and #6 misfire, #5 almost all the time. But when I check the spark at the spark plugs it is strong and doesn't appear to miss and the engine doesn't shake or hesitate. When cold it does it more and I can notice a vibration at idle as if one spark plug wasn't working well, but not as if a wire was disconnected. Compression is almost even on all cylinders except number 1 that is lower about 12%. However, spark plug number 4 looked dirty as if it wasn't burning well, but not that bad. Otherwise the car run OK and starts at the first time. Vacuum and gas pressure are normal. Any idea what may be causing this?
Thank you