It could be your cello. But it could also be your technique:
Different strings require different bow speeds and arm weights to make them sound the best. It could be that you're using appropriate weight and speed for the D string but when you cross over to the A string it becomes too much or too little. Try experimenting by practicing open strings with a fast or slow bow stroke, and with heavy or light arm weight. See what each string needs, and make sure that when you cross the strings, you adjust accordingly. In general, thinner strings need less weight and more speed, and thicker strings need more weight and less speed.
Perhaps the angle of your bow is causing you to change the contact point as you cross strings. If you're letting the tip of the bow fall towards the ground, you could end up too close to the bridge on a string crossing. To fix this, practice in front of a mirror.
Having a perfectly set up cello would probably make it sound better, but sometimes we have to change our technique to meet the needs of our equipment.