There could be many reasons, a vacuum leak is one possibility.
A miss or A collapsed engine mount or A imbalance caused by a failing component, harmonic balancers and torque converters are common.
Engine miss.
It is idling. This is hard for an engine to do smoothly.
You should check the timing and plugs.
Bad wire
Your engine probably has a vacuum leak
check spark plugs and wires
The engine
probably the governor going bad
A car that is stopped with the engine running.
The #1 cause of cold air from the heater when the car is stopped, or the engine is idling, low engine coolant levels.
asuming its a blazer around the majority of the others on this site a pluged fuel injector or a spark plug wire is rubbed and touching a ground point