Ahhh, the Motorola Surfboard SB6183. I recently had one... actually two of them, returned them in favor of an 8 channel modem because the numbers here don't lie. Consistently slow starts on that modem. The 8 channel SB6141 initiates quicker and reaches full speed quicker. TestMy.net results were very clear and totally steered me to better performance. More channels don't always equal more performance.
Read this topic that unfolded when this happened.
You be the judge, which modem SB6141 or SB6183
SB6141, still kickin' butt. Saved money AND I'm faster.
I suggest you hit up BestBuy and give it a try. Hopefully you're still within the return policy on the 16 channel modem. I recommend buying from BestBuy because of their return policy... no questions asked if you decide that it's not what you need. -- seriously, I just went through the same deal. I'm pretty sure you're experiencing the same idiosyncrasies that made me loose confidence in that modem. For what I do here, developing TMN, that kind of wildly unpredictable performance arch wasn't going to suit the job.
Let me know how it goes.
(btw, don't bother with extended warrantees, if it runs for a few days with no problem... chances are it will run for years that way. These are very reliable pieces of equipment... just don't cover the all important air vents or restrict the heat dissipation in any way. e.g. don't put it in a confined space or put anything on top of it or your router.)