Restrict Workflow to run One at a time on sharepoint list

Hi All,

I'm currently using a sharepoint list to allocate Customer call-backs (each list item is a customer's account details).

The way i've got it set up is that the user hit's "get next case" on one sharepoint list which immediately runs a looping workflow against another list to assign the next unassigned case (using a boolean true/false field).

the issue I have is that my workflow needs to to the lookup more than once to replace the permissions in that particular work item and then update the boolean value to true and if more than one individual is running that workflow at the same time, it can cause the permissions to be allocated to one individual whilst the "assigned to" field has someone else's name in it.

what i'm looking for is a way of limiting the workflow in the first SP List.

my first thought was to set the workflow to "wait for a field to =" and then a certain value with each item counting down to when it reaches the top of the list.

is this a possibility?

apologies for my poor explanation on this