Heat Creep issue(?) on E3D v6 Hotend after upgrade to Marlin 2.0

[drhender]

I have a custom-built Core XY printer with a E3D v6 Hotend with stock cooling fan, thermistor and heating cartridge and things were fine.

I put in a new main board and upgraded the firmware to Marlin 2.0. Since then, I've been having problems with failed prints. About 20 minutes into the print, the filament stops feeding/extruding. It appears that the filament is softening in the heat sink, so I believe it is a heat-creep problem. I was using a stock E3D v6 duct and fan, but even tried replacing it with a larger fan. That made no difference-- filament stopped extruding in the same place in the test print. That doesn't make a lot of sense-- I would expect a larger fan to at least delay the failure point.

I have looked through the Marlin 2.0 config files to see if maybe the Hotend fan wasn't running at full speed, but didn't find what I was looking for. Is there something there that I am missing? Any other suggestions?

Thanks in advance!!!