The place is http://www.microsoft.com/games/fsim/downloads.htm.

With scrolling down, we can find the word Flight Simulator 98 SETUP PATCH in red and also find the characters (2.9 MB), it is not the destination.
Next, we can find Aircraft/Adventure Converter for Flight Simulator 98 in red, it's the destination. You read there and will find (1515 KB), and please click above this for download it.
The name of the file in download is fsconv98.exe .

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After download it, you double-click it, and it is installed automatically. (Of course, it is the premise that you already bought FS98 and installed in your hard disc. If you installed FS98 the place not default C drive, the FS98converter automatically recognizes where to be installed itself.) Just for reference, the version of exe file of FS98 not installed FS98converter is 6.10.418, and after installation the version of it is changed to 6.10.419.

And there is a file of aafconv98.txt in the same folder of FS98, please read it. You can find the sentence rather down the page as follows.

*	Increased internal scenery buffer from 256K to 512K to fix problems with London scenery (and some 3rd party scenery as well).

That's all. The explanation is very simple in spite of importance of the matter. I don't know detail, London scenery may be heavy ??

And, FS98(J) is already released. It seems to have had a large inner buffer originally, and it is no need to apply FS98converter. (Unconfirmed information)