What good and not so expensive PSU 850W for Rtx 2080 Ti and 9900K?

Not really, that's more down to the gpu. What gpu do you have? One thing you could try is if the gpu requires 2x power leads instead of using one cable from the psu with a pair of connectors on the end swap to 2 cables from the psu and use the primary connector (1st one, not the daisy chained one) from each cable instead. This has solved a few stability problems on power hungry cards such as Vega 64 for many people. It isn't psu specific as it can happen with any psu.
 
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