Since you have a code for both over and under timing on the same cam (exhaust), the most likely suspect would be a timing belt that is slipping. I don't know all my Volvo engines very well, but the one in my 01 V70 T5 is an interference engine, which means that if the valves get too far out of sync, they can hit the top of the piston, and that will cost major dollars to fix. I would recommend that this be attended to quickly. I would also pay close attention to the tensioner, and would probably replace it as well, since this may be what failed and allowed the belt to slip. Please keep us posted on what you find when you get in there.

Jerry
01 V70 T5