Designjet 130NR stops printing magenta mid page

[PF_10]

My Designjet 130NR has developed a fault where it stops printing magenta part way through a large format page. No error or fault messages are displayed. All ink cartridges are full or close to full, all printheads are showing as OK on the display, the Image Quality Diagnostic Page prints correctly (full blocks of colour for all colours including light magenta and magenta). I have manually purged the ink tubes to be sure there were no blockages - all OK. 

If I print the page again after a short pause (time to scratch head and investigate last faulty print), the page will start printing correctly before it again stops printing magenta / light magenta. Before I start wasting money on new printheads etc, has anyone got any ideas on the likely cause of the problem or other things to try to diagnose the problem. 

Many thanks!

[Valsimot]

If the ink flow is good there is nothing else that can cause this except the printhead.