What will make it lose power all of a sudden?

Could be a number of things. Is the check engine light on? How many miles are on the engine? Is it under acceleration that it looses power or during cruising?

No the check engine light is not on. It is throwing a lot of codes tho. I know that my husband said that he can leave the store in the morning and set the cruise on about 80 and he might make it 8 miles down the road and he is running maybe 55 with the cruise still set

I would suspect a clogged catalytic converter. It's hard to diagnose a problem without data, but from what you're describing it's certainly a possibility. Catalytic converters clog over time from carbon build up. If you can, to test it, you can loosen the exhaust flange where the exhaust connects to the manifold creating a good sized gap. This will reduce the back pressure being created by the clogged cat. It's going to be loud as heck, so I wouldn't recommend driving it around town. But If the truck doesn't loose power with the exhaust separated, then you have your problem solved. Replace the cat.