That sounds a lot more serious than the standard turbo lag. I have never seen it last 10-15 seconds.
I agree, you should start with the regular maintenance items as these trucks are really sensitive towards good maintenance. Be sure you replace BOTH fuel filters. As gearitis pointed out, do all the maintenance items so at least you know where you stand. When you buy used you either dont know or you are at the mercy of believing the person you purchased from.
Beyond that, look for trends. Is it weather related, temp of truck, long drives, short drives, anything you can pinpoint at all to establish a trend. Any smoke from the tailpipe? When was the last regen? etc.