Yeah, well I have had many problems with the electrical in my 100E. I rebuilt the original Lucas generator and replaced the Voltage Regulator. but the car has no power. The generator registers over 18 volts. which apparently is due to the voltage regulator not doing its duty. Apparently I needed to Charge or Polorize the Voltage regulator. Does this sound familiar to anyone and what do I have to do to get this car going again. Signed Half power DonG.
I don't know for sure about your loss of power but you have to address your over voltage situation. I had the experience many years ago when I put a 2 litre pinto engine in a Mk2 Cortina that the engine misfired badly with an increase in revs causing a great loss of power. When I checked the system voltage it was 19 or 20 volts with higher revs, this high voltage overwhelmed the ignition system and caused a bad misfire with a huge loss of power. Try disconnecting the generator (both wires) and run the car again on the road with just battery power for a bit and see if it improves, you might be surprised? In my case when I replaced the voltage regulator the car ran great again. I do hope you find your problem.
Basically, the voltage output from an unregulated dynamo will rise directly with rpm; it's the regulator's job to limit this to about 13V to 14V. The fact that you are getting 18V says that the dynamo is doing its job, but as Brett says, you need to bring that voltage under control, otherwise you will get misfires, blown bulbs, loss of battery electrolyte and, eventually, a damaged dynamo. But the problem here is the voltage regulator.
I've scanned all the instructions on setting up the regulators, both in the Ford workshop manual and supplement. It's too big to put on here, but if you pm me an e-mail address, I'll send them on.