QoS for variable bandwidth connections

Question

My DSL connection does not have a reliable max speed. I've used Tomato's QoS in the past, and it seems to work, but chokes when the DSL speed decreases lower than the max setting.

If your upload speed varies by much, you will need to be conservative and use 90% of your lowest score.

But this doesn't mean "reserve a certain amount of bandwidth for high priority apps". It actually limits the total bandwidth of the router to whatever I put in the box. If I put a number that's higher than the actual bandwidth for that day, it can't do its job. If I put a number that's too low, it limits everything to lower than what the modem's actually capable of:

For instance, normal speedtest.net results with QoS disabled:

• 2900 kbit/s down 700 kbit/s up
• 3140 kbit/s down 723 kbit/s up

After enabling QoS at 500 kbit/s down 100 kbit/s up to prove that it's limiting everything:

• 64 kbit/s down 459 kbit/s up
• 65 kbit/s down 456 kbit/s up

I really don't want to limit my bandwidth to 50% of the max speed all the time just to make QoS work, but I do want to be able to get priority for SSH stuff and web stuff, and decrease priority for background high-bandwidth stuff. Is there a way to do this? Like when high priority packets are going out, temporarily slow transmission of low priority packets?