I went to have my car scaned, but showed to many trouble codes 10 codes in all! my problem I'm having is the engine has an intermitting miss, either at idle or rolling down the road! smooths out then

Wow....Well a leak at convertor can cause all engine management sensors to read lean mixture, then they all compensate by richening up mixture causing your problems. Start with the simple. Check the ribbed tube leading from air filter box to intake. It may have cracks open on bottom of hose you can't see from above. If it does, replace, get them at local junk yard as well as new.(Cracks or breaks when changing air filter) Check the 2 connections plugged into that same tube, make sure not leaking/have tight fit. Check all plug in connections all over the engine to make sure none are loose. Fix the leak at convertor. After leak fixed, remove battery negative cable for 2 minutes and put back on. This will clear codes fixed pertaining to leaking convertor....Now have your computer check on fault codes again. Auto parts stores do this for free! Write down all fault codes showing faulty. I imagine you will have some or all of your 3 exhaust sensors above the converter showing bad. These you can replace at about $45 each. If so Search each of your fault codes online (FIX 2000 MERCURY SABLE FAULT CODE# ) There are You tube videos on how to replace. One of the exhaust sensors is close to back of transmission, repaid from above laying blanket over engine to reach. Pull off Batt cable again for 2 min, put back on. This resets fault code and engine management system. Give it a ride to auto parts store again for code fault testing. Write it down if still bad. Two more things, EGR valve, EGR pressure sensor next. I will check back later.