In August, ambulances in the county reached 73% within target. This dropped to only 64% in December.
Ian, who is chairman of the Great Western Ambulance Service UNISON branch, says the Computer Assisted Dispatch system, is part the problem.
The system, which was put in place last September, has led to paramedics having to pull over on the way to incidents to try to get more information.
He said: "Sometimes you have only got part of an address or don't know what incident you are going to. You could be going to a stabbing or shooting. You don't know what you're responding to or whether it's safe to approach."
Ian, who is based in Bristol but covers Gloucestershire in his work, says there have been incidents around Bristol where unprepared crews have been attacked.
He added: "Information comes through on a little screen and you have to use different configurations to get the information.
"If you're driving to an emergency using blue lights and horns you have to concentrate on the road, not information that's coming through on a computer screen."
The new system also means drivers of Rapid Response Vehicles are initially sent out to wherever the call is made, which is sometimes not where the emergency is, says Ian.
A spokesman for the Great Western Ambulance Service said the organisation has ongoing discussions with staff through unions.
He said interim chief executive Anthony Marsh who, took over last September, has been going out to ambulance stations to talk to staff to raise these issues.
The spokesman added: "The new Computer Aided Dispatch system is an improvement on what we had before. It's improving response times.
"Our staff have raised issues such as accessing information. If there are things we can do to improve that then it will be done. We're quite happy to talk about that with staff and their representatives."
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Written by Citizen Gloucestershire