It dosen't sound like an air leak as any air that leaks
into the system usually.would become an air bubble that when it became big enough to stop the engine would need to be bled from the fuel system before the engine could be restarted.
However, here is the way that I have tracked down air leaks
on my Perkins
Take about a foot of clear rubber tubing and attach one of those bubble pumps that are used to prime outboard
motors from a detached fuel tank, use hose clamps for a good seal. Then attach it upstream to anything you want to check and pump fuel through it.
Disconnect the fuel line that runs from the primary filter to the secondary filter jwhere the line connects with the secondary filter and pump fuel thru the line. If there are bubbles of air in the line, move the test to before the fuel goes into the primary filter and try again. Any air leaks are quickly identified. Also check the metal fuel line that runs from the secondary pump to the injector pump
for any leakage of fuel, keeping in mind that if fuel leaks out then air can leak in.
Because your problem seems to be tempreture related, I would think that the problem is with the injector pump
and it needs to be rebuilt.
There is a possibility that the fuel line is not large enough to carry enough fuel thru the injector pump to keep the injector pump tempreture down. This usually is a concern when re-powering from one brand of engine to another, but it's something to consider.
Hope the solution is a simple one.