You are driving a car that is 20 years old. Italian cars have at best very poor ground systems. I hope you like to work on your car, cause you need to.
Starting at the front where the battery is, locate every ground wire you can, they should all be black. Once you find them, remove them and clean where they were afixed. Then reinstall using STAR washers. Do this over the entire car. It will be a Job, but in the end, you will be glad you did.
I have owned 6 of these and still own 3, been building/buying/selling Countach cars for over 23 years. I stock parts and rear tyres for them.
If I can help you can call or write any time.
My email is: JaguarXJ_S@Yahoo.com
Oh the email address, I race a Jaguar in top end events. It is faster than a Murci in the standing mile.
Regards
Chadbourn Bolles
803 532 6257