My chain slips despite new parts and tuneup
I presume all the new drivetrain parts were replaced close enough together that they were all new or new-ish at the same time. If on the other hand just the chain was replaced, the cassette or rings were highly worn, you rode anyway for some time (say a few hundred miles), and then replaced those too, it's possible you could have put enough wear on the chain in that time to keep the problem going. You can measure the chain wear with a ruler to test this.

Misaligned rear derailer hangers and/or cable housing friction can both cause this and unfortunately can be missed by shops. If that were what's happening, shifting would be poor too.

In an extreme case, rear frame misalignment can cause this. You'd probably be able to see it though.

The other thing that can do this is issues with the ratcheting mechanism inside your freehub or freewheel, which can also go unserviced at shops. The common scenario is the lubrication at the pawls gets gummy or depleted, keeping them from popping up with adequate force to engage properly with the driver or drivering (arbitrary terms for whatever toothy bit the pawls mate with). Or a pawl or pawl spring can be broken or out of place, creating the same problem. Then sometimes as you pedal, just the tip of the pawls might be engaged with the teeth, which feels solid until it slips under load due to incomplete engagement. The sound and sensation is very similar to chain slippage.

If that is what's happening, one problem is hubs can't take too much of it before the pawls or driver get damaged and can never work right again.

You could diagnose this by subbing in another wheel to test, or by getting someone to watch the skipping as it happens and confirm its the freehub turning and not the chain slipping. With experience one can make pretty good educated guesses about what's happening inside a freehub body by manually back-spinning it with the cassette off. Sometimes you hear tinkling or crunching inside and it's less of a guess.

Fixing it depends entirely on what hub it is. Usually the first step would be get some lube inside and see what happens. Some hubs make it easy to take the whole thing apart, but many do not.