I have had to change my starter on my 98 Taurus SE three times in last 1 1/2 years. I changed the battery, and the alternator a yr, and a half ago, and seven months later the starter went out again, and this week it did again. Any suggestions?

About 1 1/2 yrs ago I started having problems with my battery or so I
thought.  The car would not make a sound, no click, anything!  I brought it
to an Auto Store who tested it, and it said that I needed an alternator.  A
garage changed it, and told me that i needed to get a new battery as well
so I headed to Walmart.  Whenever I stopped for gas it did the same thing
and would not start.  After an hour of cooling down it started so I brought it
back to the garage. They tested, it, and through trial and error discovered
that it was the starter so I had to have a starter put on.  About seven
months later the same thing happened and it turned out to be the starter.  I
had to pay maint. but the starter was still under warranty.  That one lasted
until last week when it began to pull my battery down.  I went to Walmart,
and they tested the battery, and it was good.  I brought it to the same
garage, and he put the big tester on it, and the alternator, and the starter,
and battery showed good.  One week later it would not start again.  This
time no amount of hitting the starter did any good so I had to be pulled
down the street to the garage where they once again changed the starter.  
Thatrs three in a year and a half although the whole time I had my car snce
1999 I had only had to put one starter several years ago until these recent
series of starter problems.  Any suggestions or has the auto store been
selling bogus starters?.