Erasing initial gains, the Sensexwas down by over 84 points after the Reserve Bank left the key interest rate unchanged today to fight inflation and lowered economic growth projection to 6.5 per cent for this fiscal.The BSE 30-share barometer, which had added 55 points in opening trade, fell by 84.29 points. News agencies report the fall was triggered by a slide in interest rate sensitive banking, auto realty stocks.Similarly, the National Stock Exchange index Nifty was down 29.3 points at 5,170.5.