How to Clear out Water Pump Lines

[Sidenberg]

year 2000 Optimax 150. Been getting intermitten over heat buzzer but telltale and pump seem to be working fine. I will be checking and replacing impeller this week and want to make sure any debris in the lines is cleared out as well. What is the best way to do that? Blowing air? If so through where?

[boats.net]

Once have lower unit off connect a water hose to water tube on mid section.

[Sidenberg]

Causes for occasional short term overheating

Okay sounds too easy. How about this. After removing lower unit and inspecting, impeller which was replaced 2 years ago still looks new and all other pump parts are as well. Cleared out tubes and lines as suggested but no unusual or excessive debri came out except a small bit of sand.
Question is what could be causing the occasional overheat alarm to sound? When it does a quick shutdown and then restart and then it runs fine for sometimes a few minutes and sometimes the rest of the day.