Anyone has an idea of how to fix speed sensor or the wiring??

My Sr20det has a little problem that.....i think its either the voltage n vacum leak

When i start up the car, i have the rev it a lil pass 1500rpm so the dashboard light.....will be brighter as usual also when i warm up the car.....the rpm drops 700rpm or a lil bit below.....seems like battery's low but i changed it recently.... PS: i have the small battery

but if i drive the car for awhile than none of these problems will occur....does anyone have an idea??

Judging by your username, this is in a 240SX, right? If so, wrong section. This is FWD SR.