2000 Toyota Camry Vibration

Tires and Wheels problem
2000 Toyota Camry 4 cyl Two Wheel Drive Automatic 155K miles

Ever since we have had the car there has been a vibration in the front end. It did this even when we test drove the car. We had them to put a new set of tires on and the vibration went away for awhile. After about 2500 to 3000 miles it started vibrating again. Took the car in had the tires checked to see if one had lost a weight and was out of balance. They were fine. Had the tires rotated and the vibration stop again for about 2500 or 30000 miles and it started all over again. We have had everything check out that could be, and still a vibration. Any clue as what it could be? We have a "92" camry and never had any problems. I did notice the 92 has 14 inch wheels and the 2000 has 15 inch.