Howling:Timing

The Doctor goes into a box and then the Tardis blows up for reasons unexplained. Just in the box. Not earlier, not later. Perfect timing.

Why?

Weren't the engines in trouble months earlier? Damaged by the dramatic regeneration? Wasn't there a cloister bell and the Doctor had to knock the TARDIS into the future to fix them?

The TARDIS was broken then. What kept it going though the series to make it last?