2003 Ford Fairmont Pricing and Specs

This version of the Ford 5.4-litre V8 is not universally admired by mechanics. It can tend to run hot, and it’s quite common for this unit to suffer from stretched timing chains, worn chain tensioners and guides. Any problem in the area of the timing chains can lead to a misfire or stumble, and it can be worse when the engine is hot and the oil pressure that powers the timing chain mechanisms starts to fall away.

The best advice is to have the car electronically scanned. This might pinpoint the problem by throwing up a fault code that a mechanic can decipher (including the camshaft timing fault we’re talking about). Don’t continue to throw new parts at the engine; that’s a great way to waste time and money and still not fix the problem.

The battery is being drained, so you need to trace the cause to be able to fix it. You have done the most obvious checks without finding it, but keep looking. If ultimately you can't find it you could install an isolator switch to the battery, which effectively disconnects it and should stop the problem.

Those old Fords are renowned for failing body ECUs, but there would have to be an underlying problem to cause three failures in quick succession like that, so I would call in an auto electrician to sort it out.