2000 mercury sable ls v6 dohc issues

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. So sorry to hear you're having trouble with your vehicle. Given the issues you're describing, your Sable might be dealing with a combination of problems. The P0401 code typically relates to insufficient EGR flow, which can cause rough idling and hesitation. The rough idle, RPM drop with the A/C, and engine noises could indeed point to a vacuum leak or possibly a failing EGR valve or related components. The ticking sound might be related to valve noise or exhaust leaks, while the "boing" noise could be as simple as a loose heat shield or engine mount. Since the transmission stumbles when cold, that might also indicate a transmission issue, though vacuum problems could also affect it. It would be a good idea to check for any vacuum leaks first, as they can cause a wide range of symptoms. Replacing worn hoses and ensuring the EGR system is functioning properly might resolve several of these issues. If the problem persists, further investigation into the engine and transmission may be necessary. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!