1998 Rav4 engine studders occasionally while driving at low speeds, going up grades, or while stopped in gear with brake applied. No warning lights appear on dash. Car has 109,500 miles with regular oil changes.

I assume you are taking about your car or truck? I also assume you are talking about extended driving down hill as in through mountains? If you are driving down long hills through mountain areas you should be using the lower gears. There should be, depending on how old your car is, other gears other than D like L or 3L, 2L 1L. These are the low gears for the vehicle. Use them to have the engine go into higher RPM and keep the vehicle from gaining to much speed. If you have a newer car with techtonic or semi automatic shifting use this and keep the car in a lower gear. What you should see is the RPM go up and hear the engine working harder but the speed should not keep climbing. Otherwise you are going to burn out your brakes and won't have them when you need them. So try different gears, depending on the grade of the slope, till you get one that keeps you at reasonable speed without having to keep your foot on the brake. Hope this helps

it sounds like the oil sending unit which is an inexpensive part and simple to replace it should be located between radiator and engine on the side of the oil pan

If you look at the drivers side of the brake fluid reservoir you should see a rubber hose that feeds to the clutch master cylinder. The clutch master cylinder is kept full via the brake fluid reservoir.

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P0740 JEEP - Torque Converter Clutch Circuit
Symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
- Transmission shift shock
- Vehicle may not shift into the highest gear

Possible causes
- Low transmission fluid level
- Dirty transmission fluid
- Faulty torque converter clutch solenoid valve
- Torque converter clutch solenoid valve harness or connectors
- Torque converter clutch solenoid valve circuit is open or shorted

When is the code detected?
Torque Converter Clutch (TCC) unlocked for 10 sec., ECM commands converter lock-up and did not record engine speed drop.

P0740 JEEP Description
The torque converter clutch solenoid valve is activated, with the gear in D4, by the Transmission Control Module (TCM) in response to signals sent from the vehicle speed and the Engine Control Module (ECM). Lock-up piston operation will then be controlled. When the torque converter clutch solenoid valve is activated, the torque converter lockup clutch will engage creating a 1 to 1 RPM ratio between the transmission input shaft and the rotational speed of the torque converter.

Lock-up operation, however, is prohibited when A/T fluid temperature is too low.

When the accelerator pedal is depressed (less than 2/8) in lock-up condition, the engine speed should not change abruptly. If there is a big jump in engine speed, there is no lock-up.

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It's a brake warning light. Might mean low brake fluid or just mean the emergency/parking brake isn't completely released. Don't drive until you know which it is.

YOU HAVE A TORQUE CONVERTER LOCKUP ISSUE, HAVE SOMEONE LIKE THE DEALER,AUTO ZONE OR IF YOU ARE HANDY CHECK FOR CODES, NO CODES....THEN TAKE THE CAR OUT STEADY DRIVE AT AROUND 45 MPH, GENTLY HIT THE BRAKE PEDAL JUST ENOUGH TO MAKE THE REAR BRAKE LIGHTS GO ON BUT NOT HARD ENOUGH TO STOP THE CAR...DO YOU FEEL THE CONVERTER SHIFT IN AND OUT EACHJ TIME THE BRAKE PEDAL IS TOUCHED MAINTAINING A STEADY 45 MPH SPEED, IF IT THAT SAME SHUDDER YOU ARE FEELING THE TORQUE CONVERTER NEEDS TO BE REPLACED, NOT AN EASY JOB AS THE TRANSMISSION HAS TO BE REMOVED TO GET AT IT.SORRY ABOUT THE CAPS.......

Check you brake fluid, If its low when you go WOT the fluid will move away from the sensor and the light with come on and take a few seconds to reset

Gren light is never a warning , I think it's related to trany mode.You probably accidently push ed botton for transmission mode selector.
Regarding RPM it may be separate issue.

Yes, most likely a fault in the auto transmission, and the car has shut itself off to prevent further damage.

Here are the most common causes of surges, stalls at stops, low erratic idle speed, rough idle and engine hesitation, it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.