My 1996 chrysler cirrus rpm drop to zero on the freeway

The 3.5 is a great engine and in the Concorde pretty efficient for sure over 20 MPG. The 3.5 HO is 1st generation and has a 10.4 to 1 comp ratio. 89 octane is required later engines after 1998 had the compression ration lowered to 9.9 to 1 and 87 might be acceptable. Scheduled spark-plug change is usually 60K miles. If plugs have not been changed than I would certainty start with them. Check tire pressure against the driver door recommended. I expect if all else is well that at 60 MPH just under 2000 RPM in over drive and smooth..

This engine was built in Chrysler's Trenton engine plant in MI and is considered one of the best power planets at the time.

run fault codes and look for faulty transmission control module. Check that all the connections are clean /tight and dry

check crankshaft sensor.

Is it a standard? If so sounds like the clutch is giving up and needs replacing. If it is an automatic then check fluid level might be low, that can cause slippage.

sounds like a bad u=joint

you probably have a bad VSS. while driving the car see if your cruise control will engage. if it doesn't , replace the VSS. (vehicle speed sensor) It will also cause your car not to shift correctly if it is not working.

fuel filter or pump

the pcv is under the intake manifold on backside if it is leaking it can cause a high idle but i would check th ecar over for vacuum leaks as this is common as well

Is the check engine light on? If so it may be the idle air control motor that needs to be cleaned or replaced.

with a 2002 Concord 3.5 V-6, what should the rpm be at 60mph? 70 mph?

I'm not sure if the overdrive gear is lost in the tranny, but the transmission light is on.

Thanks