2005 LLY Intermittant Poor Running / Limp?

Original owner of my beloved 2005 2500HD LLY. Truck has 165,000 miles. Household use, maybe 8,000 miles towing.
I'm not a diesel mechanic, though I can rebuild British carbs pretty well.

Two issues:
For years, it has had an intermittent turbo lag. No boost when accelerating, for several seconds, then it catches, and runs right up to normal boost. (Seems like about 29 lb is max.) Would like to solve this, as it can be pretty inconvenient to try to accelerate from a stop with no boost.

Recently, I started it and got about a mile down the road when it suddenly lost power (limp mode?) and ran quite rough/"knocking". Got it home. Had it towed to an independent "diesel shop". They diagnosed it to be needing all eight fuel injectors, to the tune of about $7,000. I towed it home, As I just couldn't put that much into it right then. Since then, I've started it a few times. Seems to run fine, then after a mile or so goes limp again & rough running. When it is running well, it;s like when it was new. Absolutely fine.

I do need to use the truck, so if we're sure the injectors will fix it, I'll have that done. I really don't want to spend $7,000 and still have a problem though. I have the diagnosis sheet from shop if that helps.

Thanks,
Ed