I just got bought a 93' 300zx N/A with exhaust/ intake its got 132,000 miles

Whenever im driving and drop the rpms while moving by pressing the clutch in all the way or switching to neutral, The car will idle weird, only happens sometimes more when i step on the gas a little more then usual and then drop the rpms. But the rpms will jump around between 0-1200.. It actually will turn off at any speed could be 10 mph could be 50mph, its kinda scary when your steering locks up.. It doesnt happen when the car is stoped.

Any suggestions welcome - Thanks.