RPM gage

For some reason my rpm gage is fucked up.... when i start the car its ideling at 3 thousand and when im on the interstate or just driving around it bounces all over the place....I took it to a shop and they told me that my engine is ideling right and all i need is a new tac. If anyone knows what might be causing this please help me....Or even if anyone knows how to fix this please let me know....THANKS

When ever i race i cant even read my tac right... i need this fixed so i know when to change gears... i usually just go by the sound but you know i would liek to have my tac to go by