Head gasket replaced, now stalling

Just had my head gaskets replaced on my 2012
Subaru Legacy 2.5i and now the car stalls on me.
It's never done this before the repair. It's mostly
when I'm reversing and give some tension on the
wheel while turning out of a parking spot in reverse.
It's done it once while idling in drive. Dealership
tells me it has to relearn idle, whatever that means.
I take it back, they tell me they cleaned the carbon
from the throttles and made it relearn idle. Welp, it's
stalled 3 times since getting it back. I'm anger and
fed up, anyone have this problem or have any
advice. I feel like the dealership didn't put
something back correctly or one of the parts was
faulty. Please help.