Capping the upstream shouldn't affect the downstream, I see no reason why it should help "boost" your downstream back to 40Mb. They use separate frequencies/tones that don't overlap, so 1 doesn't impact the other.
The swinging snrm on the upstream is odd.
It sometimes goes away with a resync. Sometimes it needs the modem to be rebooted if it's your modems fault.
I recommend turning your modem off overnight, allowing your port on the DSLAM to sit idle for a few hours. Reconnect the modem in the morning and see if it's gone.
If it doesn't go away, then it's neither your modem or the DSLAM. Could be a crosstalkers modem is misbehaving, could be external noise that's making it swing. Nothing to worry about really, we see it on the downstream all the time.