hmmm strange, i tested a mission that had 200 ullysses fighters attack a SD demon. no chrash, and they where all theire. slow FPS ofcourse like 10 or something.
and have a nice pic of it too. see below

Where the ships, modded in anyway?., my current build of FS2 is a debug build, ("meaning no modded ships, and no URSA`s as they generate an error").
And gimme som information regarding your system
GFX card
and amount of RAM.
and was this a singleplayer or a Multiplayer mission
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my mission was run on a system with 256 Mb of SD ram
on a geforce 2 MMX 400
maybe you are out of memory?. FS2 will certainly Crash on out of memory and generate an Error.
there is a nice free utility for administrating cache and memory for win 9x, MX found here
http://www.outertech.com/