water pressure..please help.

Hello,
i was having problems with water pressure where you couldn't run 2 fixtures at the same time(when you had a shower you couldn't do dishes). i recently installed new air tank in home. the water pressure was great for about a month. you could have a shower, run a wash and do dishes and the water pressure wouldn't change at all. now when i turn the water on it comes out decent pressure for about 3-10 sec then drops drastically. the pressure gauge on the pump is reading 40-60 psi but there is no way that is whats coming out. the tank doesn't turn on very frequently. when the pump kicks on at 37 psi, it runs from about 45sec-1 min to fill the tank to 65 psi. i can have a long shower and the pump will only kick on once or twice. this is why i dont think its a bladder problem.

i have checked the pre charge on the tank and its 2 psi below the pumps cut in number. just wondering why the pressure ran so great for a month or so and now its back to the way it was before new tank???
thanks
would not having a pressure release on the system be an issue. usually if you have over 70 psi you need one right??