Can you post an image of your chromatograms where you are seeing this behavior? Also, the more you can give us regarding your method and your analytes, the better we can address your problem. Lots of folks on this forum have lots of experience and have seen a few things come and go.
If you are running with constant pressure, you will see broadening because the flow rate of your carrier gas changes with temperature (slower when oven is warmer).
Is it possible you're seeing carryover from a previous injection? That's a pretty long run so I assume you're sitting at a hot temperature for a while.
