RPM level

That's one sign of a potential vacuum leak, but given the mileage, there could also be a misfire problem. The Engine Computer could be trying to compensate for the drop in rpm or it could be trying to bring idle speed down to correct the results of a vacuum leak.

Start by checking the vacuum hoses for cracks or if one is disconnected. Intake manifold gaskets can leak too but that is less common. You can often find that by lightly spraying water on the gaskets while the running engine is still cold. You'll see or hear the water get sucked in and the engine speed will slow down.