The Welsh Ambulance Service is putting lives at risk by struggling to meet response targets for emergencies, the assembly's audit committee has said.
A report shows the number of calls answered within the eight-minute target fell to 47.6% in December last year.
The Welsh Assembly Government, on behalf of the service, said there had been improvements since the period covered by the report - up to January.
But it agreed more improvements were still needed.
Assembly Members said it was "unacceptable" for the service to report that an unexpected cold snap had put extra pressure on health services.
Delays at hospital emergency departments put lives unnecessarily at risk, said audit committee chairman, Conservative AM Jonathan Morgan.
"For the patient in an A&E department a lengthy wait is unacceptable. For the patient in the community awaiting a response to a 999 call the situation could be fatal," he said.
The committee recommends better management of seasonal variations in demand, more updates on response times, and a speeding up of handover times.
"The committee is concerned that targets designed to ensure a prompt response to emergency 999 calls are not being met consistently and at times, performance falls well below the required level," said Mr Morgan.
"The committee is also concerned that performance has been adversely affected by the trust's efforts to make efficiency savings."
An assembly government spokesman said the report covered June 2008 to January 2009, and there had been "sustained improvements" since February 2009.
"However, we acknowledge that further improvements are required and then will consider whether the findings are still relevant," he added.
May 2009 figures show 66.5% achievement against the eight-minute target, with 9 of the 22 Local Health Boards hitting the 60% aim.
There are also plans for more improvements, including investment in new technology.
But the public is also being urged to help reduce pressure by judging whether they need to call an ambulance or whether they can be cared for and given appropriate advice by NHS Direct or the GP out-of-hour services.
Last month the service was also criticised by a former head, Roger Thayne, who resigned in 2006 claiming it needed major investment and was putting lives at risk.
At the time Health Minister Edwina Hart said staff were doing all they could, but there were "a number of underlying issues".
These included fluctuating performance due to seasonal pressures, overworked staff and, while sickness levels had fallen, they were still too high.