Pedal slips under pressure!

[Tonio]

Pedal slips under pressure.gad derailer replaced,but still have issue on uphill or under pressure seems like pedal slips,don’t see chain move,it’s on a trek Edmonds s5,,,thanx for the help

[Joe_W]

Check for chain wear. Who ever replaced the derailleur should have done that, but still...
How: 12 links should be 12 inch. If it is more (say... 1/8) replace the chain. If it is even more, cassette and possibly chain rings need to be replaced as well. Maybe even with less wear...

Next thing to check is derailleur hanger alignment, then try to fine tune the derailleur.