If you can, pull your subs... both of them.
They may be wired in parallel:
They may also be wired in series:
Here's the math:
If they are 4 ohm speakers, series wiring will yield approximately 300-400 watts, due to the total 8 ohm load on the amp.
If they are 2 ohm speakers, series wiring will yield approximately 600 watts, due to the total 4 ohm load on the amp.
If they are 4 ohm speakers, parallel wiring will yield approximately 1000 watts, due to the 2 ohm load on the amp.
If they are 2 ohm speakers, parallel wiring will yield GAME OVER, due to a 1 ohm load on the amp. Your amp is NOT rated as 1 ohm stable.
Chances are that the anomalies you are hearing in imaging are due to the protection circuit kicking in as the amp overheats, IF they are 2 ohm speakers wired in parallel.
My money is on this horse...
Snap some pix of the wiring on the subs and the back of the magnet, once you pull them, and post them up for us to look at. The ohm value will be listed on the back of the magnets.
You MAY be able to fix this, if you rewire them (which you can easily do, with some guidance from the peanut gallery), considering you haven't done irreversible damage to the amp.
If you've run the amp too long and done internal damage, it will continue to fail intermittently and will need to be replaced. You will be able to determine this by wiring the speakers to match the amp.
Let us know what you have and we'll be able to point you in the right direction