Misfiring

Hi there, I am from Australia and I have a 2005 NF Sonata Elite with a 3.3 v6 engine. For as long as the past 3 years, a check light has been constantly on, stating there is an issue with the 02 sensor. For most of this 3 years, this has not caused any issue and a mechanic once told me, unless your fuel economy gets worse, I wouldnt replace the 02 sensor as it is an expensive job and is probably just a electrical fault. Needless to say, on pulling out of a shop last week onto a highway, i put my foot down to be able to merge properly and my caar lacked power, made loud metalic sounds and the check light flashed. Took it straight to the mechanic, my car is now misfiring on cylinder 1 3 & 5. It is shuddering at all times now and the check light is constantly flashing. I recently had the sparks replaced as per the log book servicing guidlines. I hadnt experienced this misfiring issue before. It is currently down at the mechanic, they found a small air leak in the exhaust system that they claimed could have been the caause of the o2 sensor issue, they said essentially 'suck it and see' and come back as soon as a check light comes back on. To my surprise the check engine light didnt come on within the first km, as is usually the case after having the codes cleared, with i would assume, an 02 sensor code. But my car still had a slight tinny sound and lack of power, especially noticable when the foot was put down and it changed back a gear or 2. Shortly after, the check light was back flashing again and running rought and shuddering. I took it right back to the mechanic and the same misfire codes are coming up. They have had it 2 days now and are replacing the 02 sensor, despite that error code not coming up again on the test drive mentioned. I read a previous post which talked of the timing chain slipping a couple of notches, i also saw a post where someone had Intake Oil Flow Control Valve (24355- 2G000) replaced by a hynudai service dealership and that seemed to fix his issue. I am just a little concerned that the mechanics are replacing things because they just dont know what the problem is hoping it is the cause. The issue with this is that it may not fix the problem, but I still have to pay for it and the price will begin to add up.