Fuel Pump?

I have a '90 5.0 convert mustang, all stock (for now) besides the mufflers. The fuel pump noise varys from time to time and it gets pretty loud, but other times you can't hear it almost at all. Most of the time it's pretty steady. The car just seems to lack the power a 5.0 should have, but the power seems to come back intermittently. It also starts hard, it will crank for a few seconds and start to cough and stumble until finally it seems to "catch". It only seems to do this when the car has been sitting for awhile, like after it loses fuel pressure. Although, it will start up perfectly if I prime the key to the On position a few times so the pump can build pressure. I can hear the pump whine, it is steady until like the 2nd key turn and then the pitch drops then I know the car will start up. I just need to make sure that this is the fuel pump and not something emissions related because I know I have a code for low EVP, EVR, or PFE voltage. I am going to buy a 190 lph pump off ebay if you guys give the thumbs up

Thanks for replying HISSIN50 , any help is appreciated. I don't have a fuel pressure gauge I really wish I did, I'll probably just borrow one this week to make sure the fuel pressure is low. I was also considering the fuel pressure regulator, I've been eyeing those on ebay too. You also mentioned the check valve. Is that in the pump itself so if I buy a new one that is replaced as well? Please anyone who reads this post your opinion; I'm anxious to buy the fuel pump and get 'er in there.

This is a follow up for anyone who reads this post, I'm concluding the problems and the solutions for reference. I bought the 190lph pump off ebay, which by the way was a fair price and good quality, and installed it. I found that it wasn't the check valve that was malfunctioning but there was a small hole in the hose leading from the fuel pump in the tank. Therefore after the car had been sitting for a while fuel pressure was lost. Also the power that I seemed to "lack" was justified when I discoved the SPOUT connector was dirty and damaged so the computer saw this circuit as open, which it was. When the SPOUT circuit is open the computer disables timing advance so base timing can be set. All I can say is now it feels how a 5.0 should, and before it was fixed I almost lost to a Honda CIvic. A stock one non-Si.
Thanks again HISSIN50 for your help.