Gremlins

I had the guys at TriValley Aviation (formerly maintenance express) go through the plane and they couldn't find anything wrong with the alternator or the tach.  Of course I picked up the plane, started up and everything looked fine... I took off and 5 minutes into my flight the alternator light came on and on takeoff the left tach was bouncing.  Grrr... frustrated that I now need to bring the plane back, I proceeded on home.  A few days later I setup to bring it back, and to my delight all my squawks go off on the short flight to LVK, the left alternator light comes on, the tach fluctuates on takeoff, and the left fuel gauge drops to zero.  Of course I land, go to the shop, pickup the tech and go for a demo flight where I'm certain to show him all of this and then... nothing.  nada, zilp, zilch, zero.  Nothing wrong!  the alternator light never comes one, the tach never fluctuates at all, and the left fuel gauge is rock solid.  WTF, we flew for 40 minutes, a multiple mixture and power settings and everything ran PERFECT.  Murphy's law... I have about six hours of flying scheduled for this weekend so we shall see...