Is the "MS Dynamics Navision ERP" application running on the same box as SQL Server in prod?
If so, I'd lower the fixed memory for SQL considerably.
Even if not, I'd still lower it to at least 96G (98304MB); at 128G, Windows itself needs some RAM to manage the other RAM.
The only non-memoty things that would normally ever slow it that much is:
1) I/O issues
2) tempdb issues.
So I suggest verifying those first before looking at other things.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.