Hi
I don't know if this is related or not, but I saw last night that there is a Simoleon Shredder
http://www.insimenator.net/viewtopic.php?t=204 which causes this exact phenomena you are describing. It deletes simoleons based on a percentage of the funds, so it would seem to be a random amount.
I'm wondering if somehow you have downloaded something that has this hack in it?
Eric mentions he might incorporate it into Insiminator. I don't use Insiminator so I don't know if he did that or not. Perhaps that is where the problem lies?