I can't get my aftermarket boost gauge to work?

It was already installed by previous owner, but wrong. I got it hooked up correctly I thought. It even worked correctly for a little while. Now it does 3 things: goes crazy and fluctuates all over the place, stays at 30 psi, or stays at full vacuum. Also when I turn the headlights on to change the illumination color it makes it go to full vacuum and sorta twitch. I am almost positive my connections are good. I know I have a good ground. I don't know how to check the sending unit to see if it is good, but I have a multimeter. the boost gauge is a Prosport electrical boost gauge.