I think I know this one. I just had the same symptoms on my 2000 1.8 Zetec Focus. I narrowed it down to the VSS (Vehicle Speed Sensor). When it fails, because it goes via the ECU, it not only drops the speedo (and often some of the other dash displays) it also confuses the ECU, resulting in a drop in power (if you keep your foot on the accelerator) and a complete cut out of you clutch. Sound familiar? The sensor itself is not expensive and in theory it should be easy enought to replace as a DIY job... it's should be just a case of removing one roll pin and pulling it out! However, because of it's awkward location and the pin being extremely difficult to extract (on older cars), I would recommend you get a garage to do it. It's likely they will break of the top half in the process and will take several hours extracting the remainder, so make sure you agree the price before they start. The new Speed Sensors have been modified to make the job easier. I speak from experience... I've just been through this scenario. Fitting the VSS resolved the problem.
Hope this helps.
Good Luck.
Chris..