I'm using canon XTi and

I'm using canon XTi and newly acquired tamron 24-135 3.5-5.6 lens. I had the lens for a bout a week and yesterday while shooting out side in the bright light at high speed i noticed that my camera would freeze up during autofocusing and the view finder will turn from bright to gray as though the camera was going into standby. Few seconds later it will regain the function in terms of it focusing and metering ability just to have the problem repeat itslef few shots later. Of course the problem disappeared once i showed it to my friend and have failed to duplicate this while shooting inside the building. When i was ouitside i have tried every setting, every zoom length, every shooting mode to no avail. Twice i had the error message requiring me to shut down the camera and "reboot". Any idea what that was and should be sending the lens back for servicing? Or perhaps it is third party lens compatibility issue. Thank you very much

Steve