Gifts for visiting other cities.

While I do buy a lot at the trade centers I have found that the visiting mayor bubbles appear to pop up at time intervals I live in GMT -7 (USA Mountain Time) and have noticed that bubbles are everywhere starting about 1am. I get a lot of bubble items from places I DO NOT purchase from. .I go trolling from bubbles every night starting at about 1am and there are bubbles everywhere. Now I am also noticing there are other similar times in the day but I have not been able to pin them down completely. They appear to be somewhere between 3 to 6 hour intervals. I will try to pay closer attention and report back. Again, when I am trolling for mayor bubbles I am not always buying. But I do agree with a first come, first serve basis on who gets them. I just try to be first.