Midi disappearing in Logic 8

Hey guys. i have been working with Logic 8 for about a year or so and have never had this issue up until about a month ago, but it seems to be happening more and more frequently and I am pretty much at my wits end because it is costing me a lot of time and totally ruining my work flow. I am currently running Logic 8 on a 6 month old imac. What is happening is that, I'll work on a track for a week or so and randomly boot up my computer, start Logic and some of the instruments, that I have chosen will change sounds or not even make any sound at all, as if I never even recorded. This happens with 3rd party synths and Logic's very own built in synths, normally when I record a line i will bounce it into an audio file to save cpu, but in some cases I leave it as midi in the event that I want to maybe come back to it later to program some automation, variation, etc...This is really frustrating because I can spend hours or even days sound designing, come back later, and the sound doesn't even exist anymore. This has happened to me twice now, do you guys have any suggestions?

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http://www.logicprohelp.com/viewtopic.php?p=312139#312139