Diesel genearator problem?

I have a Yanmar 4kv diesel generator which is fed from the boats diesel tank. After it's been running for a while (the time varies) the generator starts to die away and give off a little blue smoke, after it dips in power it then comes back to full running, after a while this will happen again. Problem is when it reduces in power and comes back it trips the electrics. What is making it lose power temporarily?