Hello,
I posted a thread on supraforums regardin gthis, I had the same problem, and I have a fix.
I guy on this forums tried it too and it worked very well.
Disconnect the Matrix hoses from the firewall, and try flushing them both ways with a high power jet wash. (Use hose extensions else your bay will be well messy)
If that fails to fix it, the only thing I could find to un clog it was Nitrox Drain Cleaner. It is 99% Sulphuric acid so be very careful. You can buy it from B&Q and its about £6.
Run some down in there and leave if for about 30 mins, shoudl start to bubble nicly, then rinse through with a hose pipe (not high pressure) from the side you put the cleaner down.
Repeat this 3-4 times in both directions untill water flown cleanly.
Then Use jetwash again to 100% clean out any acid and crap that is in there.
This worked fine for me and continued to work for months untill I have now taken my car off the road.
Seems alot easier than changing the matrix!!!!
*** Be care full with that drain cleaner , I use welding gloves when using it.
***** Flushing aditives didnt do squat!
Martin