2000 Pontiac Grand Prix

My car has been shaking for about 4-5 months now. Once I hit 20, it shakes violently and then it smoothens out a little at about 35-40 and shakes again after that. Now I had some 18" rims on my car so I felt every bump. I have done several things to try to fix the problem. First I went to go get the wheels aligned and they had told me that I needed to get new engine mounts and that was the reason why my car was shaking. I took it to get new engine mounts installed and it still was shaking and the mechanic told me that I needed to buy a new tire for my front right tire. I did that and it turns out my rim was bent so I had my old rims and tires (I've had no problem at all with those) that I had on before, put back on and it still shakes. I took it back to the same mechanic and he said that he thinks it's the struts but I might want to get a second opinion because that is the only thing he sees wrong with it. If I drive real slow with the car, it feels like I'm driving with a flat tire. Any other ideas of why it does this?