1999 Ford Taurus Intermittent Starting Problems

I've been having starting problems with this vehicle for over a year and it makes running errands impossible. The car has sudden death syndrome and phantom windshield wipers (wipers seem to be most common problem in this type of car.)

Just driving to and from work is fine, but idling seems rough. Lights dim a little and brighten up again when I hit the gas. As long as the car is on, it won't typically die on me, unless it's 90+ degrees outside.

The real problem is when I drive the car for 15+ minutes, park, and try to restart 10-30 mins later. It will attempt to start, but then the RPMS will fizzle out before I can get the car moving. Subsequent starting attempts, become less fruitful. It turns over, but won't catch. If the car gets a break for a couple hours, it will start fine. Jumping the battery sometimes works and other times it doesn't.

Took the car in the first time, and mechanic couldn't find any problem, and said maybe replacing the spark plugs would help, because they needed to be replaced anyways. Didn't change spark plugs, because it wouldn't guarantee to fix the problem.

It finally died while I was driving on a hot day. It just started to struggle while idling at stop light and fizzled out after about 15 minutes. Waited too long to take it in, and battery was dead by that time.

Mechanic traced the circuitry and found a draw on the battery from gem module. Couldn't replace gem module bc Ford doesn't make part anymore. They unplugged gem module, cleaned connectors, and replaced battery. Draw seemed to go away. Cost me $350.

Three days later, drove the car for 15-20 mins, and parked. Tried to restart 15 minutes later and had the same problem. What's going on? Could it be an alternator or spark plug problem? Or do I have a wacky electrical issue that's causing the battery to die?