1993 Buick Century 6 cyl Front Wheel Drive Automatic 220000 miles

car is breaking down on acceleration but not when engine idleing. problem began some time ago and less pronounced then, only in wet weather and driveability returned within a short time. weather has turned very cold and wet and car is barely driveable now. the onset was not at initial start but after driving a number of kilometres can you nail this down for me. car has a distributorless ignition. motor stumbles and shakes, very little torgue.

Check
it for tune up
check if all coils firing
check timing chain and gears