A bit of background information (I'll try keep it brief): Last year I bought an old but unused bike, 5 speeds with internal gearing.
Apparently the shop bought a lot of bikes somewhere in the 90's (not sure), but never got around to selling them. When I bought it, it was wrapped in plastic, had been stored in the shop's stock house for about 20 years and free of corrosion.
I'm about 95 kilos, thread quite hard but rides exclusive to paved bike paths. The original, 20(?) year-old wheel lasted me a year with no problems. The front wheel is still fine and true.
Spokes breaking - and getting replaced
After a little under a year, I suddenly noticed that a few spokes had broken. On closer inspection, quite a few were too loose. Should have noticed sooner but didn't.
I took it to a shop, where they advised me to have the wheel rebuilt which I paid them to do. The gearing being internal, the new rims and spokes were built on the existing hub.
After just two weeks, the back wheel suddenly began to feel wobbly on my way to work. As careful as I could, I drove the bike back to the shop. Almost all spokes were terribly loose.
They retensioned the wheel free of charge (of course) and sent me on my way. The following weeks, I periodically checked that all spokes were still tensioned.
2,5 months later, I noticed three of the spokes were broken close to the hub. Went back to the shop and had the spokes replaced (free or charge). 1 month later I noticed 2 broken spokes and had those replace as well. They seemed less eager to keep fixing the wheel free of charge, and when I asked why the spokes kept breaking, the guy muttered something about the hub holes maybe had burrs due to wear.
Now, a few weeks later, I find another spoke broken.
My gut tells me this all stems from a bad build, that quickly lost tension and thus damaged the spokes. I find the explanation about a worn hub a bit far fetched, but I don't have the knowledge to dismiss the theory.
Is there any way this is not the shop's fault? - A bad build? Cheap spokes? Improperly tensioned?
Given the wheel's history, is there any point in keep replacing spokes, a couple at a time, or should I get the wheel rebuilt (preferably at the shop's expense)?
Could the hub in any way be to blame for this?
Update Oct 24th
I've been trying to get in touch with the manager of the shop throughout the week. Failed again to reach him this morning, so I figured I'd have a chat about my problem with one of the guys on the floor.
Tried to get him to provide at least a theory of why my spokes keep breaking, but not much came out of it really. He mentioned that he've seen, on rare occasaions, that a worn hub could cut the spokes (could be the same guy as I spoke to last time).
I'll check the hubs as soon as possible, as @Daniel R Hicks suggested. Due to plumbing work in our appartment, I haven't been home or able to check my bike all week.
If I don't see any indicatations that the spokes were put in the wrong way, I'm going to follow the advice most of you have, and take my bike to another shop for advice and repair.
Thanks so far! - I'll update you after I've payed the other shop a visit.
Update Nov 3rd
Took the bike to the other shop, told them the story. Let them decide to replace the broken spoke or them all.
They decided to replace just the broken spoke with a DT spoke (what ever that means). They also trued the wheel, which had gotten a slight "eggy" shape.
My fingers are crossed that this wheel will last now.
Once again, thank you all for your input!