1999 buick regal

i changed the 3 spark plugs in the back since they are the harder one-so now my car hestiate while going up hill and about 50 mph-when i first replaced them didnt have any problem til couple days later-is this a trans problem or tune -up problem123000 miles?

What brand of spark plugs did you use? Also, with 123K miles did you replace the ignition wires? Quite common to damage the plug wires when you tug and pull on the rear bank wires to get them off. Ignition wires should be replaced at around 100K miles. It is highly recommended to use the AC Delco Premium Silicone 7mm wires as replacements.

What brand of spark plugs did you use? Also, with 123K miles did you replace the ignition wires? Quite common to damage the plug wires when you tug and pull on the rear bank wires to get them off. Ignition wires should be replaced at around 100K miles. It is highly recommended to use the AC Delco Premium Silicone 7mm wires as replacements.
I used ac delco plugs 6.99 apiece iridium it drives alittle better-only now kinda shiver around 55 mph i put 1/2 pint of sea foam FOR TRAN in trann and seafoam for fuel also replaced other plugs