Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
This is caused by a temperature issue inside the CVT unit. When the internal temperature of the CVT reaches 280F the MID display shows "Slow Down" and remains on that until the temperature of the CVT fluid has dropped. During that time period throttle response will be significantly reduced as well, and usually once you come to a stop the throttle will lock at 5% maximum application until the temperature drops.
This could be in relation to internal issues inside the transmission (high friction will increase temperatures... also worth noting that if the fluid has been changed or topped up with the wrong fluid it will cause high friction and this problem), though it could also be environmental. If the vehicle is driven in particularly hot weather and at sustained high speeds it will cause this condition as well. If the vehicle regularly sees high speeds (extended high way speeds or higher) we often drop the fluid level about 1/2 quart to compensate for this. The reduced level allows more ventilation inside the case (And still allowing without the safe fluid level range) and the fluid remains cooler preventing the warning from coming on.As long as nothing has happened with the CVT fluid (no change or fluid or topping off), I would suggest getting the CVT scanned to see if any faults were recorded the last time it happened, provided it has been recent (They only retain for a day or two). If no faults were set to indicate an internal problem that may be related, I would inspect the fluid condition, and if the mileage is high enough (30k or higher) I would change out the fluid with fresh Mitsubishi CVT (there is NO acceptable substitute) but reduce the fluid 1/4-1/2 quart and compare behavior.
I will be in and out throughout the day; if you need more help with this just let me know.