A couple of weeks ago I returned to sieve at PPR and now I'm getting credits from my work the way I expected, smoothly. The only problem is that, for some strange reasons, the RAC assigned by Primegrid is way higher then the average credits I get. For example, just now, I got a RAC (in my account page) of around 4.8M while, in the last ten days, I started with around 1M/day going up to ~3.5M.... see this http://boincstats.com/en/charts/11/user/gainPerDay/8810/chart.png

This is just a little bit annoying, all of my teammates keep me asking which server-farm I rented....

It is known "feature" when PSA credit is added. Used to be much worse when is was added at once.
It was a reason to change that and now PSA is added in batches acording to your RAC. Once first batch of PSA is added, RAC goes up a bit and next batch gives up even more PSA credit as you have seen in your graph.
There are ~10M outstanding and it will clear in 3 days.
Eventually, it RAC will settle down again.

This is similar to sitution when many task would be done off-line and returned just before deadline and/or when bunch of WU gets validated at once. RAC would jump...
____________
My stats

ok, thank you for your explanation.