If you are pretty sure that after being repaired, the camera started such problem, then report it to the repair center, and get it fixed. Such repair center has to have a kind of warranty in labor and repair services. If there is nothing you can achieve from this, then you can try in install the firmware on it, and see if this helps.
Prior to it, try in reseting the settings to the factiory configuration. If it does not work, then follow the below recommendation.
This firmware upgrade operation is eay to accomplish, buit I have to warn you that if there is a hard problem with your camera, such process may be aborted in the midle of it, and your camera will be useless completely. Think carefully about it.
If you still want to try it, you can get into Canon web site and follow the instructions of the following link http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=314&modelid=11154
There is always the option of send it to a qualified repair center to fix it (different one from the one you used).
Good luck and best regards,