Class 10 these days is pretty much a marketing term. you can have a class 10 SD card that costs 15 dollars and one that costs 80$. it all depends on the speed of the card. Sandisk 30mb/s are very good and anything with higher speed than that functions really nicely. Lexar i hear isn't too bad but kingston and fujifilm tend to have lower grade cards
it can also be due to overheating problems. If you're recording for long periods of time (especially in the summer) it can do that.
good luck and upgrading your SD card is a definitely a good investment.