Why did my trx300ex jump time

I picked up a 300ex for around 350 bucks and it dident run. It was out of time so I set it back in time and it ran great. Then after riding it for a little bit it jumped time and got weak spark at the exact same time. I replaced the coils and it regained spark but I still cant figure out why it jumped time. The chain and timing chain tensioner look good so I cant figure out why it jumped time. Also the timing chain is plenty tight but not to tight. Any idea why it jumped time or how I can prevent it from jumping time in the future?