New bike is slipping gears Got a bike 3 weeks ago. It now slips gears WITHOUT shifting. only slipping when I pedal hard or uphill.. It shifts good. I weigh 250lbs.

edit: with a new derailleur
Even if a derailleur appears to shift well, it could be out of adjustment. My shifter cable recently snapped and I replaced it. I put on a new one, and after adjusting it to the point of the shifting being "good", there was still some skipping when pedaling up a steep hill; some finer adjusting took care of it (while also improving the shifting from "good" to "excellent").

You mention that the derailleur is new, which suggests that the bicycle isn't. A used bicycle can have a worn out drive-train.

If the chain slips on the cogs, a new derailleur won't fix it. If the chain is slipping on the rear cassette, both the chain and cog need to be replaced. If it skips on the front, then the chain and front rings will have to be replaced, and likely the rear cassette also (because the new chain will quite possibly skip on the new one).

A second-hand bicycle doesn't have to appear to be well-used to need drive train component replacements. A bike that is used daily for year-round daily commuting can need a new cassette and chain well before it's two years old, especially with lower-end components, even if kept clean and well-maintained.