Alright, I've been toying with this issue for months now, and thanks to you guys I've now apparently resolved it. Thought I'd share my story and observations in case they shed any light:
Firstly, some context: I'm on a Verizon Employee account (i'm not the employee, but a family member is an engineer...sadly she can't share any real info with me in this regard). I got my Thunderbolt in March, so I have Unlimited Data and from the very beginning I've had access to the stock Mobile Hotspot, have been using it constantly and have never been charged. For the last several months I'm averaging about 25GB of data on my phone per month; as I use it for World of Warcraft, Netflix, Torrents, and massive amounts of random data from classwork in medical school.
My connection has always been relatively reliable, but I have ALWAYS had some kind of noticeable filter in place when tethering. However, it DOES NOT flat out disconnect me and I've learned to work around it. The way this filter appears to work is that I can have constant data running to my tethered desktop PC, but as soon as the bandwidth exceeds about 200 kilobytes per second for more than about 10 seconds, the filter will kick in a boot my connection. It DOES NOT disconnect me, and as soon as I refresh or attempt to reconnect to the server it resumes just fine, but with the same limits. I have tested this thoroughly by monitoring bandwidth from WOW, Netflix, and though setting limiters to bandwidth in uTorrent. As long as I stay below 200kB/sec, I have a steady non-dropping connection, and everythings fine. TO clarify: I can get consistent acute bandwidth of up to 2.5 MBit with Speedtest.net and I can get downloads to reach about 800-1000 kB/sec on a pretty regular basis so there's speed to be had, HOWEVER, running at these speeds for more than about 10 seconds ALWAYS results in this network boot (that again, does NOT drop my 4G conenction, and does NOT affect my wifi connection).
Recently I had theorized some of this might be due to bad reception, as I can never get above 3 bars at my house, and often spontaneously drop to 2 bars. I had considering buying a 700mhz signal booster, antenna, etc, but all in all it costs about $500, and I might as well give in to Time-Warner cable internet at that point.
Finally after reading here and various other sites I came to a solution to get around this that is working great. Again, I'm a noob in this, so bear with me.
I rooted with a very simple rooting guide from Revolutionary: http://www.thunderboltforums.com/forum/htc-thundebolt-hacking-guides/5259-rs-approved-revolutionary-5-minutes-less-root-without-data-loss.html
I tried using Open Garden, Barnacle, and some other random tethering programs that shouldn't be traceable by Verizon, but I now believe they ARE. These did not change the connection pattern in any way.
So, I went to the tether program you described above: Wireless Tether for Root Users. This app is no longer on the Android Marketplace under Verizon, so you have use a side loader, this appears to be as good one:
Android Central Sideload Wonder Machine v. 1.2 - Android Forums
I loaded the newest WTRU 3.1 Beta6: http://code.google.com/p/android-wifi-tether/
NOW, I have a completely unstoppable connection!!!
I'm getting 7-9Mbit on Speedtest (faster overall acute d/l speed) and I'm not hitting any kind of bandwidth cap that kicks me, steady connections to Netflix, WOW, etc. And this is on 2 bars of signal.
Thank you guys so much for the suggestion to use WTRU, it seems to have resolved the problem entirely. (No wonder Verizon banned it from their marketplace.)