How many decimal places are carried in the computations? This could be a source of the variability in the optimal portfolio results even when many iterations are run. Is there a way to increase the number of digits carried? (Of course this would be expected to increase solution time significantly, but then users could test to see if they get better consistency of recommended portfolio and extra patience is worthwhile.)
Also with regard to decimal places, a comment. For use with daily data, risk free rate should be displayed with more than two digits, otherwise rounding obscures the intended number in 2012s low risk free rate situation. Right now I am running trial version, maybe in paid version one can change the digits beyond decimal point.
The cell formatting can be changed to see more decimal places. Moving to double precision will require changes to the VBA code. We can do this and send you a modified version upon request. This will increase the memory load but should not significantly reduce processing time.