The fuel pressure gauge is highly recommended.

Usually if you sell something that isn't running well, you take a hit financially. MIght as well fix it IMO.

Could be that you are sucking air into the fuel system (usually a poorly installed primary fuel filter cap will cause this) or an injector is slowly leaking combustion gasses back into the fuel system. There is a video out on the "bubble test" that will show you if that is happening. I will look for the link.

A scangaugeII will help you troubleshoot. Look for the FICM parameters (MPower, VPower, FICM sync, etc)