Opposition parties have criticised Health Minister Edwina Hart's second review of the Welsh Ambulance Service.
The latest review must be completed by the end of the year
She said the latest review, to be done by the end of the year, was needed to examine improvements and reassure people public it will get better still.
But Liberal Democrat Jenny Randerson said too many reviews were creating a backlog of decisions.
Conservative Jonathan Morgan also warned too many reviews meant that "you get tied up in knots".
The latest review will focus on performance and look at how the service is progressing its modernisation plans.
It has been called a year after a critical report by the Wales Audit Office, which found that a lack of direction and leadership had led to poor performance
Ms Hart told AMs that, despite progress, there was still "considerable variation" in ambulance response times.
The latest official statistics, for September, showed the ambulance service was exceeding the target of achieving 60% of emergency responses within eight minutes.
But it failed to meet all its other key targets, including ambulances for life-threatening category A calls.
The trust also failed to get 95% of ambulances, rather than lone paramedic first responders, to the most serious and immediately life-threatening calls.
Ms Randerson said she had supported Ms Hart's policy or reviewing health matters since she took office "up to now".
But she said: "The ambulance review is in a different category: the ambulance service has already been reviewed by the auditor general, reviewed by the assembly committee, there are two current reviews involving the ambulance service and you have got to let time pass to allow the fundamental changes to bed in".
She warned the latest review "had to be done very quickly and not get in the way of their core work, because the trust is pretty overstretched anyway".
Ms Randerson said the assembly government had "to move very soon onto stage two, which is when the reviews come in and where results are".
"They've got to be integrated into the day-to-day working of the NHS and that is going to be upheaval on a lot of different fronts, considering the large amounts of reviews that the minister has commissioned."
Conservative Mr Morgan said: "Ultimately we need to look at a whole systems approach: it's not just about numbers of ambulance and paramedic crews, it's about the interface between the ambulance service and hospitals.
"It's the fault of hospitals in south Wales, in particular, that we have ambulances queuing.
"When you see ambulances queuing at the A&E department you always think it's the ambulances' fault. Well, it's the problem within the hospital."