I just installed a silver faced monster tach with shift lite and it intermittently goes down about six hundred RPMs when idleing or when the engine revs up. It's not so much a flicker as it actually swings back 600 rpms and then returns to normal. When it's operating correctly, it reads as accurate as the stock tach until it swings back in rpms. There is no pattern to when it intermittently swings and all of my connections are firmly soldered together. I also made sure I had clean power and ground sources. I tested the tach on my Mustang and there's nothing wrong with it. I'm wondering if there is something wrong with the tach adapter. Has anybody experienced this? Thanks in advance for your help.