Bizarre Slow Query Problem (again)

I previously posted about a problem where it seemed that changing the
case of the word "BY" in a SELECT query was causing it to run much much
faster.

Now I've hit the same thing again, where basically almost any change I
make to how the query is executed (so that it still performs the same
function) causes the performance to jump from a dismal 7 or 8 seconds
to instantaneous. It's a very simple query of the form:

SELECT Min(MyValue) FROM MyTable WHERE MyKey = @0

which was running fine until a moment ago, when it suddently started
running hopelessly slowly. If change anything in the query to
lowercase (or the Min to uppercase), it runs fine again. Last time
someone suggested something about a bad plan being cached, and after a
bit of research I found the commands DBCC DROPCLEANBUFFERS and DBCC
FREEPROCCACHE. Sure enough, after running these, the query started
running fine again. The question is

a) why is this happening? Is it a bug in my code, or in SQL server?
b) is it worth detecting it and fixing it automatically? I.e, should I
put some code in that notices that a query is running far too slowly,
then runs "DBCC DROPCLEANBUFFERS; DBCC FREEPROCCACHE"? Or will that
cause other problems?

Thanks