If they are jumping out of gear you are probably going to need to replace at least two gears and possibly a shift fork. 

DO NOT KEEP PRESSURE AGAINST THE SHIFT LEVER WITH YOUR FOOT IN ATTEMPT TO HOLD IT IN GEAR. 

Trying to hold it in gear burns the tip s of the shift forks. 

I hate to say it but you need to park it or fix it before something breaks.....then it will be real expensive.