Acceleration decrease and engine shaking

My 2002 had a shimmy last weeks during high speeds after buying new tires. Took it in and they found all the wheels were out of balanced. They checked boots, rods etc and said there were no loose parts and it looked fine outside of the balance. It did fine for a few days and now it is shaking worse during acceleration both at higher speeds and low speed. It also has moments of deceleration during hard acceleration. I have replaced the lines but not the plugs. Could this be a spark plug issue or something else?

when you say you changed the "lines" i assume you mean you changed the plug wires?

how many miles are on your GP? is this a 3800 or a 3100?

Yep, replaced the wires when a similar issue showed up a month ago and it helped that so I didn't bother with the plugs. It is a 3800 and is at 120,000 miles.

when this shaking happens, is it usually when the engine is under a load...for example climbing a hill, or passing? this would usually indicate a missfire

i would assume there is no SES light?

with your mileage, i would recommend doing a good tune-up if it hasent been done in a while. change the plugs, and fuel filter and see what that does.

Thanks! I am hoping it is that simple. It feels like a possible misfire to me so I will work it from that angle and see if it helps. Thanks for the response!