Why does my lawn mower stop driving while mowing?

Is this one of those continiously variable speed transmissions?

Not the single-speed-at-a-time shifters.

Then it could be fluid loss at the transaxle.

(Just like transmission fluid)

I've never had to add fluid to mine, but I'll look tomorrow.

If it's not that kind of transmission, then the problem would be a lot simpler.

Drive belt is old/loose, along with possibly old/missing idler pulley spring.

Another thing to check, as I had this happen to me.

There is a safety switch on the brake linkage that gets surrounded by grass clippings and dirt build up.

The switch connector is on the right side, closest to the engine, under the fuel tank.

I had to remove the grass mat that had built up, and took off the connector and blew it out with air.

Apparently, being out in the rain or washing the mower got the grass mat wet,

which kept water around the connector and shorted the contacts.

Mower stopped dead while mowing, and I thought the transmission was bad.

Turned out to be the switch, which was still good.

I love 0 cost repairs.