I have a 98 Volvo s70 with about 135,000 miles on it. Recently I noticed that when cruising on the freeway (5th gear, maybe 2800-3000 rpm) my engine temp tends to drop about a quarter warm or sometimes down to zero. If I go a few miles in 4th it will warm up again and sometimes stay and sometimes drop back down. Any ideas? I can't imagine my engine is actually cooling down but I have no idea where to start.

no year, no real help.
mike cruises at 90 mpy (Dallas new speed limit 85 on one road)
larry at 75
me 45. so>>>
but doing it backwards works way better
what is speed at 3000 RPM , a common speed.
answer in my 96 goes 59 mph. in 5th gear.

the rpm is a function of what tranny and tire size.
here are all listed
The G16 is no V8 it spins fast and last 300k miles. if serviced.
see this table for 3000 rpm speeds.
there are 5 speeds possible in top gear.
what transmission tell you which it is , there are 3 trans. and 2 engines. each are Unique.

http://www.fixkick.com/part-time-test.html#SPEED

chart one is stock. tires.
chart 2 is big tires.
the equation is there so you can do yours, from any RPM you want.

The engine may misfire and/or buck while driving at freeway speeds with no fault code stored in the powertrain control module (PCM). This can be caused by an intermittent fault with the camshaft synchronizer assembly. This condition can be diagnosed by monitoring knock sensor data as per Ford service bulletin #05-22-12. Replacement of the synchronizer assembly will be required to correct this concern.
https://repairpal.com/engine-buckmisfire-at-freeway-speeds-934

I hope this helps

The synchromesh is worn on the 5th gear.
Hold it down or swap in a replacement gearbox.

You maybe low on clutch fluid. (possible leak)
Check fluid level, if low add more and pump the clutch.

Sounds like an Exhaust Manifold leak on only one cylinder. The Gasket is probably blown out. Check around where manifold is bolted to the engine. Any leaks are usually evident by a black sooty deposit.

The most common cause of intermittent problems is a poor ground connection.Check the engine and the chassis grounds for tightness and clean surfaces.

56k just about due for 60,000 mile srvice. When you take it in have the problem reviewed. Good uck.New trans fluid might do the trick. Good Luck, Ned

the transmission is probably on its way out. you might not have noticed when you test drove the car that it was running high rpms but youwould notice it on the highway.its probably on its last legs though

shift tranny from 1st-3rd if engine revs up clutch needs replacing

You should have another gear "5th" at 70mph your tacho should read about 2500 rpm.
Make sure you don't need to depress something or push the gear stick against a spring to select 5th.
Is there a gear layout on top of the gearstick knob, do you have 5th gear on there?
The highest gear ratio is a little over 1 to 1 so at 70 mph you would expect 3500 rpm, but, it is a little over 1 to 1 so it will be closer to 2500rpm.