Does it work Ok without the antenna extension cable?
If so, it could be the cable that's at fault. I know this is stating the obvious but might be worth a try.
Assuming that it doesn't work ok without antenna extension then I would suggest trying a different antenna. GPS antennas are complex things and do fail from time to time.
Would be surprised if it is a fault in the unit itself - if one channel had gone down then could understand it, but for every channel to cause a problem is unusual.
Good luck!