I use this
http://www.amazon.com/Motorola-Surfb...f=cm_lmf_tit_8 and it works fine for me.
The SB5101 is good too but costs twice as much as the SB5100 and has no noticeable difference, unless I'm not savvy enough to recognize it.
Sometimes my modem goes through weird phases where there's a problem like you described and I have to mess with the various cables and splitters. I think it has more to do with my lines outside than the modem, but I'm too lazy to call a tech out to look whenever there's a problem.
Also, I have the Performance package, so speeds aren't crazy fast where I need a hardcore expensive modem to utilize it (if that's really a factor).