I have a weird situation, where simple queries seem to never finish

for instance

SELECT top 100 ArticleID FROM Article WHERE  ProductGroupID=379114
returns immediately

SELECT top 1000 ArticleID FROM Article WHERE  ProductGroupID=379114
never returns

SELECT ArticleID FROM Article WHERE  ProductGroupID=379114
never returns

SELECT top 1000 ArticleID FROM Article
returns immediately

by 'returning' I mean 'in query analyzer the green check mark appears and it says "Query executed successfully"'. 

I sometimes get the rows painted to the grid in qa, but still the query goes on waiting for my client to time out - 'sometimes':

SELECT     ProductGroupID AS Product23_1_, ArticleID AS ArticleID1_, ArticleID AS ArticleID18_0_, Inventory_Name AS Inventory3_18_0_, 
                      Inventory_UnitOfMeasure AS Inventory4_18_0_, BusinessKey AS Business5_18_0_, Name AS Name18_0_, ServesPeople AS ServesPe7_18_0_, 
                      InStock AS InStock18_0_, Description AS Descript9_18_0_, Description2 AS Descrip10_18_0_, TechnicalData AS Technic11_18_0_, 
                      IsDiscontinued AS IsDisco12_18_0_, Release AS Release18_0_, Classifications AS Classif14_18_0_, DistributorName AS Distrib15_18_0_, 
                      DistributorProductCode AS Distrib16_18_0_, Options AS Options18_0_, IsPromoted AS IsPromoted18_0_, IsBulkyFreight AS IsBulky19_18_0_, 
                      IsBackOrderOnly AS IsBackO20_18_0_, Price AS Price18_0_, Weight AS Weight18_0_, ProductGroupID AS Product23_18_0_, 
                      ConversationID AS Convers24_18_0_, DistributorID AS Distrib25_18_0_, type AS Type18_0_
FROM         Article AS articles0_
WHERE     (IsDiscontinued = '0') AND (ProductGroupID = 379121)

shows this behavior.

I have no idea what is going on. Probably select is broken ;)

Anyone can tell me how to handle such a situation? More info, anyone?