86 F250 diesel not starting (again)


I bought a 1986 F250 diesel pickup last fall and it ran fine. It sat all winter and I fired it up once in February during a warm spell. When I tried to get it going in the spring, the starter sounded slow and bogged down (and the pickup didn’t start). It did this for a while until finally, the starter didn’t even turn over. I would just hear a click from the starter relay when I turned the key. I thought it might be the battery or cables so replaced the 2 batteries, got a new positive cable and new negative cable ends. Nothing changed. Finally took it in and they said the starter was bad and replaced it. Started great for about 10 days. Then the starter sounded slow and bogged down again (and wouldn’t start the pickup). Then, again, nothing but a click from the relay. Thought I might have gotten a bum starter, took it out, they tested it and, sure enough, the starter was bad. They gave me another one and I installed it. Started great for about 10 days and now it’s not turning over as fast and the pickup doesn’t start. What is going on? Is there something in the chain of starting events that might be ruining my starter? Or am I just getting bad starters? Or could they have put in the wrong starter?