Faulty Crank position sensor. If not it possible  the cam position sensor. 

What is the mileage? Any check engine light or codes? Has the fuel filter been replaced? Maybe try some fuel injector cleaner to see if that helps. Using a scan tool and looking at the live data may reveal your problem.

Try replacing the gas cap and clearing the light. The gas cap may not be the problem but the light means there's an evaporative emissions leak and the system isn't properly pressurized.

I would expect trouble codes from a bad crank position sensor and other symptoms like rough idle. It probably wouldn't be intermittent. What makes you think it's that?

There are a lot of fuel injector cleaners out there that are a better choice than Sea Foam IMO. For some reason Autozone seems to like promoting that stuff. Check the internet for yourself but Chevron Techron Concentrate Plus fuel system cleaner has a good reputation. I have a suspicion your fuel injectors need a little TLC at that mileage.

When looking at the troubleshooting section of my Chilton manual for 94-04 Mustangs, fuel injectors are a common link between hard starting and intermittent stalling, other than the things you've already addressed. Again, live data from an inexpensive scan tool will probably tell the story, if it's a bad crank pos. sensor or whatever else is going on.

I would recommend getting the known issues straight first and go from there. Find something other than Sea Foam like yaser says. What's you're mechanics opinion about the crank position sensor? I would think he has already done a diagnostic scan.

Never take you vehicle to Firestone or any of the chain stores.  They will rip you off and mess up your car.

It’s got 117k miles on it. The fuel pump and filter and relay were all replaced. It’s not throwing any codes other then my fuel cap light is on. It’s not idling rough either. I thought it could be my crank shaft position sensor but I’m not sure. I was recommended sea foam from autozone.

I will definitely replace the gas cap. And I have just been doing some research and the crank shaft position sensor was one of the things that could cause hard start. I’m not exactly sure what to do I’ll definitely try some fuel stuff like sea foam but I’ll look for better stuff online. I started my car up today and it has gotten me to work fine. I had the clutch replaced two weeks ago but when I got it it felt very smooth and light to the touch. Now it feels a little more stiff I’m not sure that could be a problem my mechanic told me whoever did the clutch may have messed it up just based on what he has seen. He did say it drove fine and that it never stalled or had a hard start for him. 

Well it’s a family mechanic and he only checked the fuel pump, alternator, battery and starter. He didn’t find anything and said it started up for him every time and never stalled but for me the next day had a hard time starting up 3 times. I was thinking about just going to Firestone and having them diagnose it but I don’t want to spend any more money, currently don’t have the time or money to put into the car. I greatly appreciate the help. If it would be the fuel injectors is any fuel additive a quick fix or should I look at having them replaced. I’ve been able to fix a few things on my own I’m my garage I’m not sure if fuel injectors are hard to replace or not.

Okay I’m not sure where to take it then I would like to get a diagnostic just to see what’s going on with my car. I love my car and I know the 03-04 are super reliable, well as far as sports cars go. But I’m not sure what to do weather to sell it or maybe put money into it to fix it.

Try a new gas cap and some Chevron Techron Concentrate Plus fuel system cleaner and hope for the best. That won't cost much.

The fuel injectors aren't difficult to replace but OEM injectors are expensive. They shouldn't need replacing yet, just cleaning.

I would avoid a chain shop like Firestone unless you have past experience with them and trust them. You could buy a scan tool for what they will probably charge for a diagnosis. I use a $30 scan tool on my 2001 Mustang GT and it sees everything the car is capable of showing. Good luck.