Sql-server – Cannot create an instance of OLE DB provider “OraOLEDB.Oracle” for linked server

When you delete from heaps, allocated space may not be unallocated unless you use a table lock. See "Deleting Rows from a Heap" from DELETE on MSDN

This is a separate problem to fragmentation (which happens of course)

Just in case, have you tried this to update usage info?

EXEC sp_spaceused 'MySchema.MyTable', 'true'

With no indexes or keys, you can't defragment it normally.
You can add/drop a clustered index, or do something like this

SELECT * INTO [MySchema].[MyTableNew] 
FROM [MySchema].[MyTable]
-- ORDER BY something

-- if you like
EXEC sp_spaceused 'MySchema.MyTableNew', 'true'

DROP TABLE [MySchema].[MyTable];

EXEC sp_rename 'MySchema.MyTableNew', 'MyTable';

oops: just seen updates, my Internets has been broken on and off...

One reason could be somebody changing options or running sp_configure. That would be logged in your error log.

Please read this article: Using Sp_configure To Change a Value Will Issue DBCC FREEPROCCACHE

using sp_configure to change a configuration value causes SQL Server to issue a DBCC FREEPROCCACHE statement.

and

The same behavior will occur if you use a GUI such as SQL Server Enterprise Manager that issues an sp_configure call that changes a parameter.

The Plan Caching and Recompilation in SQL Server 2012 white paper gives some other possibilities.

Flush Entire Plan Cache The following operations flush the entire plan cache, and therefore, cause fresh compilations of batches that are submitted the first time afterwards:

• Detaching a database
• Upgrading a database to a later database compatibility level
• Restoring a database
• DBCC FREEPROCCACHE command
• RECONFIGURE command (many of the options to this command cause a flush)
• ALTER DATABASE … MODIFY FILEGROUP command
• Modifying a collation using ALTER DATABASE … COLLATE command

Flush Database-Related Entries in Plan Cache The following operations flush the plan cache entries that refer to a particular database, and cause fresh compilations afterwards.

• DBCC FLUSHPROCINDB command
• ALTER DATABASE … MODIFY NAME = command
• ALTER DATABASE … SET ONLINE command
• ALTER DATABASE … SET OFFLINE command
• ALTER DATABASE … SET EMERGENCY command
• DROP DATABASE command
• When a database auto-closes

Lots of these events should be in your log files.