Definitely getting her teeth checked is the first thing to do. She may be choking on a "cud" of hay because she has difficulty chewing it thoroughly.
However, based on what you've said above I suspect she *really* likes getting the hard feed in the winter time and tends to bolt it in excitement.
That's great that she's on grass half the year and you only have to deal with this when feeding concentrates in the winter.
I will leave specific feed recommendations to other posters, but there are lots of other things you can do to deal with a food bolter.
Wetting the feed is an easy one, feeding small quanitities throughout the day rather than one big feed is another but may be impossible for you. Tack shops and catalogs here in the US sell a feeder with a wire ring installed around the inside; it keeps the horse from opening its mouth all the way when their head is in the feeder so they have to take small bites. A really low tech method to acheive the same goal is to put rocks or blocks of wood in the feeder so she has to eat around them, again, forcing small bites. A narrow, deep bucket rather than the traditional flat feeder helps as well.
If it's feasible for you with your schedule, I would leave her in her stall with her hay for a period of time before feeding her the concentrates as well. 20 - 45 minutes with her hay *may* blunt her urge to bolt her feed.