If you disable both CnQ and C1E there should be no controlled reason why the CPU is throttling.
The issue is the VRMs. The 4-phase VRM design (based on an old AM2 design and not so optimized for AM3 processors that normally make use of the split +1 or +2 phase to provide a separate NB voltage/output) in the M4N68T-M V2 doesn't have a very good track record attempting to push 125W processors and over-current-protection based throttling is common in order to deal with VRM overload and overheat and prevent failures. You can expect to have performance issues until you upgrade to a board with better VRM capacity.
Look to a new board with better VRM capacity - going from 4-phase to a proper AM3-enabled 4+1 with the split +1 phase for the NB can make a big difference. Ensure that the VRMs are cooled - through-airflow from the stock CPU cooler can provide this.
If you don't have money for a new board (yet), consider a big undervolt - and an underclock as necessary to maintain stability. It's better to get the constant performance of 2.6Ghz with a significant undervolt than to have 3.4Ghz sometimes and 800Mhz sometimes.
You can read more about the issues on VRMs and why to pay attention to these issues through the link in my sig (About VRMs & MOSFETs) - there've been worse cases in which VRM overload (i.e. not enough capacity to provide the power) have caught fire.... none on the ASUS brand though, due to the presence of over-current protection.
Edited by xd_1771 - 2/3/12 at 12:18am