Problem with 2 unequal WAN Gateways

[Speeedymauss]

Hey Guys,
im new in the pfsense system and have some trouble with my WAN gateway setup.

My situation:
i have two wan gateways one 400/20 and a 50/10. the faster one is my default gateway and i configured a gateway group for failover (i don't want any loadbalancing). The faster one is tier 1 and has a weight of 30. The slower is tier 5 with a weight of 1.

my error:
everytime when i start a speedtest on a client my slower gateway loses the connection to the internet, the dsl connection shuts down. i have no idea why.

maybe someone here with an idea or suggestions what i should look for? for me its a very strange behavior and i have no explanation…

thank you