Nope, not seeing this. 

I wouldn't call that a stall on the cpu, either--I have seen BOINC apps that really stall--cpu usage drops to 2-3%, everything else is dead slow, and Task Manager is needed to kill the app.

I can't say I remember this about RieselSieve, but it's not real unusual for projects to release apps that checkpoint themselves badly or not at all--they look like a restart in the progress (time elaspsed, pct done, time remaining), but it's just the stats lying.  There have also been apps that estimate completion time badly, also showing wacky numbers for progress.

Personally, I don't watch them that closely.

If it keeps happening, trying resetting the project, and if that doesn't help, detach and attach again.