Many times people complain about transmission problems at highway
speeds. Most of the time it's either a partially plugged fuel filter, or an
ignition misfire that causes this. The ignition misfire will cause the
shudder you speak of. If you have not replaced your fuel filter in the past
couple of years, do so now. If you have not replaced the spark plugs and
wires in the past 4 years or longer, do so now. Replace both the wires and
plugs. I have experienced carbon tracking on spark plugs and wires. If you
only replace one, the carbon track will transfer to the other new component.
Yes, I'm a GM mechanic.
These are the culprits to eliminate before you take your car to a
transmission shop. In any case, you will have a tuned engine, and then if
the problem persists, you can have the transmission looked at.
There is no way to change transmission shift points. These are computer
P.S. It would not be unusual to have one bad ignition coil on your vehicle.