Question: 2000 Chevy Malibu 3.1 V6. What would cause bucking when I start off its fine it changes no problem i feel it shifting very smoothly even on the highway it's OK except when trans in that 4th gear and hit a hill or grade and it goes to shift back to 3rd gear it bucks a few times then it fine again.

When I'm driving around the city sometimes between 1500 rpm or about 50 to 60 km's i get the bucking then when I get over that speed it's fine again But it intermittent . I changed the spark plugs with A/C platinum also the plug wires NGK /// changed the fuel filter had the mass air flow sensor checked and cleaned Had it checked its showing NO codes no engine light on there is 130,000 KM's I'm the only driver and I take very good care of it just put a muffler on it 2 front struts car is in very good condition even though it a 2000 . thanks.

Answer: Well, if you don't have any engine codes or drive ability problems that set the check engine light on, you could possibly have a transmission problem. There could be an issue with the shifting, or the torque converter lockup. You'd really need someone experienced with transmissions to test drive it and give their opinion on the shudder and bucking. It does have quite a few miles on it, and even with proper maintenance you could still have trouble. It would need some checking into by a technician.

Thanks for your response  it was the number 4 cylinder misfire but it only happened some times. Like i told you it was intermittent. It was  broken porcelain insulator on the plug it was spit down the side it was losing fire through the porcelain out the side. My son in law had the same thing happen to his Chevy pickup after he put in new plugs. Only had the plugs in about a week and had to replace 2 of them. I guess you can get bad plugs even though they are new. So the set that i took 1 of them  was the problem but when i replaced them never though it could be one of the new plugs  but it was thanks again and have a great day.