my tach has gone haywire

Hi meesky. If you haven't been in the cluster or there's no reason to believe the needle has been touched you are probably looking at a faulty RPM sensor. They aren't a particularly expensive part but installation can be tricky. Take a trip to see the parts counter guy for your local Ford dealership and ask for a price for the sensor and pick his brain for installation. You can probably determine from that conversation whether you want to tackle the job yourself or not. The guy at your local parts store may be a source of help as well but you'll probably do better at the Ford dealer. Also, a trip to your local library may yield a Chilton's for your T-Bird which will give you the procedure as well. And it may have a troubleshooting guide to help you make sure that's where your problem lies. Good luck!