My 2002 V6 (it has 125k on it) had been running pretty well up untill recently. whenever it gets to about 40 mph it starts to shake untill it shifts into another gear. Also, whenever i accelerate it sputters and harldey has any power. when im going a constant speed it doesnt have that problem. Changed the Spark Plugs already so its not that.. any ideas??

 

are you getting a check engine light? if so, that would be the easiest way to figure it out. i had a similar problem recently in my 94 GT and it turned out to be bad O2 sensors.

 

What did the plugs look like when you pulled them?

 

the check engine light cleared when i replaced the spark plugs. So theres no code to diagnose.

 

Most likely a clogged cat.

 

check fuel filter and pump. I had my 98 do roughly the same and couldn't figure it out for the life of me, The pump was kinda half working. replaced and had no problems since. Rust can build up on the bottom of the tank and the screen will get partially plugged and restrict fuel flow. Just use a gauge and test @ the fuel rails.

 

fuel filter
fuel pump (may work but not give enough pressure)
clogged cat
bad plug wire (have seen them get worn due to bad placement and cause similar issues)
check plug gap
coil pack

even if the SES/CEL is off there could still be a code as only critical ones actually trigger the light.