This is what I have found from my experience. I dropped D2 for over 4 years and just came back a month ago. On the very first day I got a 15 hour temp ban...doing NOTHING. Then a few days later doing a Trist run I got a temp ban..not that these runs were that quick either which was infuriorating.
A couple of days after I was doing a Baal run in NM, the guy accidently makes a Norm game, so I exit to enter the next one and get another day ban.
After that though I have not received any temp bans at all. Even more quite a few times I have jumped in and out very quickly with nothing happening. In fact a couple of days ago I helped a friend mule items. He was jumping in and out like nothing you have seen before and nothing happens.
Sadly the real reason behind these temp bans is unknown. Sorry but I just do not care to believe Blizzards official reasons, at least at face value. From my perception it seemed that they did a few temp bans to me at the begining to kind of warn me to "play nice", i.e. keep me from being a problem. It is like they want to show new players they have the capabilities of banning us at will.
I suspect that if Blizzards reasons are true, the real truth maybe that they have cut down on the number of hardware being used to save money.
Well whatever the reason you can try this. If you get into a game and have to jump in and out, trying being still for a short bit. Most people have told be that being still for half a minute to a minute prevents this. Seems to work for me when I mule items