Good afternoon,
We have problems!
We have a 150 Watt Chinese red and black laser. The tube is not brand new, but appears to be working. We have replaced the mirrors with copper mirrors and are still having this problem. After about ten minutes of cutting (we cut primarily 1/4 inch mdf) the cuts start to not go all the way through. This is also when the carriage and nose piece are get hotter than they should.

Things we have already tried (multiple times)
* We have aligned all three mirrors (replacing the 2nd and third with copper, the first by the tube is a Mo mirror) we have them dead center.
* We replaced the painters tape with some thermal label paper on top of the foil tape and noticed it gets hot (leaves a mark) all around the opening and spreads to the carriage.
*Our industrial water chiller is working just fine, keeping it cool at 18 Celcius.
*we checked and cleaned the lens (we do regularly) and it is clean and appears unbroken.

Problem suspected- is this a split beam, how would we check and how would we fix it....?
Could it be something else?

Thanks for any and all advice!