I have a 2000 Isuzu Rodeo and my vehicle can barely get over 20 mph and sometimes stalls and turns off while sitting.

Could be the idle sensor or maybe a vacuum leak.

Replace the fuel filter and check for fuel pressure.

your shifter cable is going bad replace shifter cable.

Engine may be idling to high. Check for vacuum leak.

Check the fuel pressure it could be the fuel pump or pressure regulator.

check to see if you have a idle air control valve

You need to replace the speedometer gear that drives the sensor.

try cleaning the throttle body with cleaner and a rag,sometimes carbon builds up and restricts airflow and can cause stalling,just hold the throttle open with the engine off and spray a little cleaner on a rag and clean the black sludge build up with the rag over your finger,hope this helps

the sensor on the gearbox needs replaced common problem with that age group. sensers are around 20-30 quid hope this helps?

Take of EGR valve and clean it and put it back on