I think your problem is that the 760 does currently not support burning 99min CDs. According to the supported media list only 90min media are supported: http://www.plextor.com/english/support/760_Media_List_CD-R.htm , ("TDK 48X 91 min CD-R") as compatible. However there is a chance this might be added with a coming firmware update. I'd recommend contacting Plextor support and let them know your need.
It is possible to use 99min media with Gigarec 1.4x in the Premium (and likely in the new premium 2 too). The Premium supports 90, 94 and 99min media. This will result in 136min22sec capacity (look here: http://homepage2.nifty.com/yss/premium/premium.htm and search for "136min22sec").
Edit: If i remember correctly 90/99min media always are identified as 80min, simply because 90/99min media do not officially exist by CD standards. So you're always overburning a '80min' CD whose capacity is determined by whether it is a 90 or 99 min media. (someone correct me if i'm wrong on this).