You've searched for all things but you've neglected one thing in the search and that is to change and check the wires and pipes of fuel an gas. The problem is occurring due to these two things. So you should repair them first. You can see the information about feul relay by clicking on the following link
www.all-parts.com/diagnostics/mechanics/Mercury-Sable.html
www.all-parts.com/diagnostics/mechanics/Mercury-Sable.html
