Great beer, but sluggish ferment

Guys,

Take a look at this grain and mash bill:
5lb 8oz of Muntons Maris Otter Pale
3lb 0oz of Weyermann Munich 10L
1lb 4oz of Muntons Crystal 45 Malt
8.00 oz of US Caramel 120L Malt
4.00 oz of US Caramel 10L Malt

Mashing in at 153F. Over 60 minutes temp drops to about 148. Typical mash efficiencies are between 80 and 82%.
I get 6.7 gal of 1.047 pre-boil wort and 5.5 gal of OG 1.054 after a 60 minute boil.

Yeast is WYeast 1098 British Ale.

The beer this malt bill makes is awesome, but the fermentation is dog slow.
It usually takes about 14 days to get from OG 1.054 to 1.019 and there it gets slow. It sometimes takes another 21 days to get to terminal gravity around 1.014.

I'm tried various methods with the yeast:
- Two packs
- One pack with a 1 quart starter
- Two one quart starters with re-used yeast
- One pack in a 1 quart starter and a fresh pack pitched at 14 days

Any suggestions here? Wort too dextrinous? Mash temp to high? Too low?

I'd love to speed this up somewhat.

That looks like over 20% crystal malt. That may not be the issue, but that's what sticks out to me.

Also, check mrmalty.com and look at the "yeast pitching calculator" to make sure you're pitching the correct amount of yeast. I suspect there is something with the yeast health. With an OG of 1.054, and yeast dated from one month ago, the correct starter size would be 2.65 liters.

Are you keeping the temperature of the fermenting beer constant (at what temp?) during the entire time or do you let it warm up after the initial fast ferment? Cold yeast work slow.

I start it at 68F and try to hold it there approximately for 14 days and then let it go to wherever it wants. Most of the time it rises to 70 for a few days and then falls back to 68 - 67.

I'm not that concerned with yeast health because I've made 1.090 imperial stouts with this yeast strain (mind you with those big dextrinous beers always finish around 1.018). I never re-use yeast after it's done an imperial stout.

Perhaps more aeration?