general problems of the 2005 Honda Civic

On 2/26/10, I had 4 new tires installed in my 2005 Civic; at 56,000 miles. Even purchased the warranty. About two weeks later, I felt a very mild vibration between 48-52 mph. I didn't make such a big deal of it then. The past 6 weeks or so, about 60,000 miles, the Civic's vibrating has gotten worse between 48-52mph. I took it for it's maintenance, had everything done, especially the break rotors and alignment. The vibrating has not stopped. I brought it back to the dealership. According to them, the car was checked, and felt the vibrating. Car was re-aligned, the said it was the rear struts. I had them replaced with a different auto mechanic, then had it re-aligned once more to another auto mechanic. Still vibrates. Brought it back to the mechanic who installed the rear struts. He did a test drive, and balanced the tires. The vibrating is still there. Now it's between 48-60mph at random. Sometimes it does it, sometimes it's mild, sometimes it stronger, depending after my apply the brakes. The vibrating occurs under the seats, floor, rear, some is felt very mildly on the dashboard. Could be since the rest of the car is vibrating. I'm very frustrated.

(1) new vehicle - steering was not as responsive as previous vehicle (same model) (2) during a left turn motion of engine caused skid into right wall of underpass. (3) left front engine mount was replaced - I have the original mount. Vehicle had approx 4700 miles when accident occurred.