1994 4.0 automatic, When put in gear it starts bouncing back and forth from 400 rpms to 700

[clapped4.0]

Anyone know why it does this? It just revs back and forth.

 

[rumblecloud]

Could be a few things.
Have you pulled any codes yet?

Vacuum leak
O2 Sensors
MAF sensor
And more...

Normal RPM should be anywhere from 8- 1000rpm at idle Higher at cold start-up, then down as it warms. Have you performed any maintenance prior to this happening?
What's the mileage?
Does the rpm continue to rise and fall when it's warmed up?

A cheapo OBD1 code reader from a local auto parts store could offer some clues as to what might be causing it.

Please post back with more info so we can help.