I just gave my TS-808 pedal another test out a few minutes ago and it is still footswitching reliably, so, I'll call it well and truly fixed now.
Who would have thought that one little 10nF ceramic disc capacitor would cause all that trouble?...but there you have it folks, funny thing is that the capacitor looks perfectly normal to me, that explains why the cause of the footswitching issue was difficult to find, faulty components don't always look like they're faulty.
Here's a pic of the faulty cap which caused the unreliable footswitching issue:
So how did I determine it was faulty?, I used my digital multimeter set to it's highest resistance range (2000k, or 2M) to measure the DC resistance of the cap, which in theory should be infinite, when I measured it I got a reading of about 1.8M, I compared that reading to the reading I got when I measured the DC resistance of the yellow cap (a known good one) that I used as a replacement, and got an infinite resistance reading, I then soldered-in the replacement cap.
In the schematic I posted, there's a 68k resistor going from +V,this resistor is connected in series with a 22k resistor which connects to circuit ground, using my digital multimeter, I measured the DC voltage at the point where the 68k resistor and the 22k resistor connect to each other (it measured 2.28V), there's also a 1M resistor connected to that point, and this connects to one of the contacts of the mechanical footswitch via a 22 Ohm resistor, I found that the DC voltage on the 22 Ohm resistor side of the 1M resistor was measuring about .5v to about .8 v when the footswitching was not working properly, now what's really interesting is that when I removed the faulty cap, the voltage at that point of the circuit read about 1.6v, so, my conclusion was that something about the faulty cap was reducing the voltage when the cap was in-circuit, I can now understand what was going on, the 1.8M DC resistance of the cap in series with the 1M resistor was forming what's called a "Voltage Divider" in electronics, normally the DC resistance of the cap should have measured infinite, but, it was actually measuring 1.8M, the 1M resistor is acting as a "Pull-Up" resistor for the mechanical footswitch, when you press down on the footswitch it causes the normally 1.6V on the 1M resistor to drop to 0V, this acts as a trigger-signal for the rest of the footswitching circuitry, because the normally 1.6V was being pulled-down to about .5v by the faulty cap, the rest of the footswitching circuit wasn't being triggered properly, and seemed to be stuck in one state, pressing down on the mechanical footswitch had no effect, as soon as I replaced the faulty cap with the new yellow cap, the voltage measured 1.6v again, and that restored the footswitching to normal operation.