2003 Holden Rodeo warning light comes on

The dash 'check transmission" light occasionally comes on in my rodeo.

When this happens the car goes into a "limp" mode where it only runs in low gear.

If I stop and turn the engine off then on, the light clears and the car drives normally.

The transmission had a service about 40,000k's ago, so I assume the oil is ok (no leaks visible).

Any ideas what might be going on, and is it safe to drive

Thanks