Why such high run time ? There's very little need to have a zone run for 60 min.. Of course this is all dependent on design.
What kind of controller do you have?
Is this only one a particular zone?
Check your water meter and calculate your water usage when the zone is running fine ( It's probably in cubic feet) And then when the zone barly works. Does the usage seem the same or when it barly works is the water meter barly moving?
When the zone looses pressure- does any other zones come on around the yard? Some older controllers- if it's programmed in on a second program it will kick on another zone while the other one is still running. New controllers just stack programming and revert back to the second program after the 1st one has finished if they are overlapping.
If it's a particular zone, Id focus on the valve too. It could be failing or that the flow control is turned down just enough to adjust by itself and eventually shutting it self down.. So possibly replace the valve