Hold light flashing on the dash

Engine Performance problem Two Wheel Drive Automatic 85,000 miles

Hi

We bought a Mazsa Familia 2 months ago from a car dealer. Every so often when driving the hold light will flash on the dashboard. I have taken it to the car dealers mechanic and they said they checked everything and they cannot find any problem. I drove off and it started happening again so I left the motor running so they could see because as soon as you turn the motor off the hold light stops flashing. The mechanic thinks it might be something to do with the speedo sensor. Does this sound right? I am worried that it will be something to do with the transmission and if it is not picked up now while the car is under a 3 month warranty we may have a big bill to pay later.

Any ideas?

Thanks