My 1998 terios revs up and down when stationary by its self

Any codes? Could be TPS sensor.

this will be on the carberattor small screw on the pad the is connected to your cable on your peddle

I think you should check the fuellines and for sure the injectors,these are most probably blocked.It can also be the fuelfilter that is clogged
Staert by replacing the filter and try to start.Once its starts leave it running stationary and remove the fuellines one by one from the injectors.If you loosen the first one and the engine rpm go down a bit you know the injector is good...tighten the fuelline again and move on to the next one and so on.

The spark plugs are high up on top of the engine. You will probably need to remove the air filter box to get access to them. I'm not sure if the box is bolted down or a 'pull off' type. They are covered by the plug shaped coil packs. These just pull out and push back in. The attached photo has them marked with a red circle.

Check the half shafts for worn CV joints is the 1st thing. Also so you think it may be a misfire, they usually happen in higher gears when engine load is highest.

could also be u-joint or axle joint bad

the engine is fine- what you are seeing is the transmission fluid flowing fast enough in transmission to cause the speedometer to move- does the transmission slip at all? might be time to have transmission serviced- fluid and filter change.

something must be grounded on your instrumental panel, try replacing the relay or fuse.

It is either the cable that goes from the pedal to the throttle body, or the throttle plate is sticking.

I have seen this symptom with a broken motor mount. The engine torque lifts the engine and pulls on the throttle cable. When the engine comes to rest, all goes back to normal