I was unaware that the 2010 ST model had a DPF, but then I'm often surprised by unexpected things (like the guy in the mirror when I'm shaving, I'm sure I've seen him before somewhere, but he copies my movements really well so I'm not going to complain).
It COULD be a DPF issue, but it could also be a boost control solenoid. If this plays up, the turbo won't boost and you'll have clouds of black smoke around (or you should, but the DPF might be capturing this and filling up). It would make sense to check boost levels. Boost should be 0 at idle, but should climb to around 20psi at 1800-2000rpm (the peak shouldn't be too much higher than that, maybe 22-23psi tops).
If it's not getting boost, there are several possibilities.
1) Turbo's shot. I doubt it (for now).
2) BCS is shot. Test it: connect the turbocharger directly to the vacuum pump (max boost at all times). Drive the car but try to back off the throttle gently, otherwise you'll end up with turbochatter as it overboosts. If the power is there, it's probably the BCS (a common fault). Either replace the BCS with a new unit, or move to a Dawes or Tillix valve.
3) Vacuum leak. Unusual for it to be erratic but it's possible. Usually it's a hardening of the hoses allowing them to crack particularly where they bend, sometimes the hose will gloss up on the inside and allow vacuum to leak.
You might also consider cleaning the MAFS (in the air intake just outside the air filter box). Do not under any circumstances put anything except liquid or gas in the hole in the end (you'll see it). Electrical contact cleaner - or MAFS cleaner (same stuff) will help a lot here.