The crank arm slips when pressing hard on pedals with Shimano Nexus
I'd start with a new chain as a simple maintenance matter - this should have been done when the new rear cog went on anyway.

Then compare how your new chain lies around the front chainring compared to the old one.  It should mesh fairly well over the whole length of contact, not having phased waves of contact and spaces.

Finally, are you positive its the front chainring skipping?  Your feet are on the cranks so it can still be the back, and you feel it through the pedals which makes it seem to be the front.   Get a friend to ride along side, or try riding on a treadmill or rollers... or even use a gopro and a mount to see what's going on.