Everytime i get to about the mid game (500-750) usually about the time when the AI has researched the interdictor/CC-7700 my game always crashes due to large fleets clashing its always the large fleet fights has anyone else had this happen to tem? if so how did you fix it?

I can't say I've experienced this in the current form of this game but had it many years ago. The only way around it, if possible is to either move your fleet or check if theres another mission running that may create the crash too.

I havent had issues with the larger fights specifically but Reb does have a memory leak built into it. This may be a symptom. Save early, save often, and rather than going back out to the galaxy map LOAD that new save instead. Doing so flushes the leak. Make it a habit every 50 days or so and before you know a big fight is coming and see if that helps.