Cranks, has spark and almost starts....what could be the problem?

What is your question?

My 1990 GMC has given me so many problems for the last year. So many parts have been replaced! Now it cranks, has spark and almost start and if this makes sense it will start for 1/2 a second. Have replaced the starter 3 times in the last 2 weeks......what would make the starter go bad that often? Please help! Thanks!