I just bought a 1995 mustang GT with 100K miles. It is having an intermittent starting problem. I can always hear the fuel pump run when I start the car. I attached a pressure gauge to the fuel rail. When I turn the ignition on (but not to start the car), the fuel pressure usually goes up to 35 psi and stays there. However, some times the pressure goes up to 35 then immediately goes to 0. When the engine is running, the pressure is at a steady 30 psi. If I pull the vacuum hose off of the fuel regulator, the fuel pressure goes to 40 psi. I am not sure what to check next. The fuel regulator looks like it has been replaced (it is a Bosch). Also, I am not sure if this is related, but when the engine is under load (like going up a long hill), I can hear pinging (or an exhaust leak) coming from the passenger side of the engine. When this occurs, the CEL comes on temporarily. I checked and it is code 172 for a lean O2 sensor on the passenger side. Any help would be appreciated. Thanks!