An A330 crew was distracted while dealing with an air conditioning system malfunction. There was a track deviation after the FMGC reverted from a heading mode to the NAV mode.

Narrative

On climb-out; aircraft was vectored off the SID and given a 350 heading and a climb to FL350. The heading and altitude were set in the FCU and verified on the FMA by the Captain and First Officer. This heading closely paralleled the SID. Later; during the climb; it was noted that the air conditioning system was sending below freezing air to the cabin. As the Captain worked the problem; and the new First Officer helped and accomplished his other duties; the aircraft somehow reverted to the navigation mode. This resulted in a turn to the right. The crew and ATC noticed the turn at about 010 degrees and an immediate correction was made. This aircraft was dispatched with two MEL's on the air conditioning system. Somehow the combination made temperature control problematic and the distraction contributed to the delayed recognition of the turn. While I am still not sure why the aircraft reverted to the NAV mode; it should have been recognized sooner. I needed to remember the cardinal rule of one pilot flying the aircraft exclusively and the other working the problem. Secondarily; the compound MEL's led to a distraction during a busy phase of flight. This contributed to the delayed recognition. These MEL's had existed for three and seven days each and the aircraft had transited Maintenance stations multiple times. Although legal; MEL's that could cause distractions should be repaired in a timelier manner.