Hi Beth,
What you are describing is usually caused by a mismatch in configuration between the two devices. Auto is a setting that must be used on both devices for it to work. This would be the preferred setting. You obviously know how to set it on the switch port, but you must also set it on the Sonicwall IF. If auto is not an available setting on the SonicWall, then you will need to set both sides manually. They must match. This includes speed, duplex and flow. If you have a tool such as NetSight, you can inspect the switch port for both sides capabilities and find the best match. If not, you will need to reference documentation for each device. If that is not available, try setting the switch to 100Mbps, Half duplex, No flow control. I suspect the duplex is currently the mismatched item causing the speed issue. If one side is Full and the other Half, the link will just crawl. Of course you will also need to confirm all the pieces and parts are good. A bad cable or interface can cause trouble as well.