the tachometer of my 2000 si stops working when i floor it and vtec starts, it freezes around 5 or 6 thousand rpm, but the engine works properly, after a while the tach starts to work right, sometimes it takes like one or two days to return to normal, anyone knows why

 

it sounds like your tach is going out on you if it was jumping around I would say it is altenater. It also could be a bad ground or loose conection somewhere. I would check all your connections first.