Strange Millenium II Behaviour

[davent]

Day 1. Saturday finished wiring up a very old project that had been sitting in a drawer for years, an MXR Envelope Filter (totally underwhelming but that's for another day) with a Millenium II switch. Fired up fine and the switch did it's thing as expected. As i played around with the pedal seeing what it mostly wouldn't do the LED slowly dimmed down to nothing, disengage the pedal then engage again and the LED would give a very brief faint glow. The LED finally stopped lighting altogether.

The Millenium was put together using a 2N7000, one 4148 & one 400? diodes.
Tried it with a battery as well as a power supply. Next two days just used the power supply.
The MXR has no pulldown resistor on the output.

Took readings in it's non-functioning state;
LED Anode 8.86V, Cathode 7.2V.
MOSFET Drain 7.2V, Gate 10mV, Source 0V.

Day 2. No fiddling's been done but tried it out again anyways, the LED is back but now it quickly ramps up to full brightness, turns off immediately when told to. No further messing around.

Day 3. Today (Monday), it's been on for the last 10 hours and has functioned without issue whenever tested during that time, LED immediately on/off when switched. Nothing has been changed with the pedal since i finished the wiring on Saturday but the switch's behaviour has been transformed for no apparent reason.

Readings for the LED are now 1.8V, anode and 5-10mV for the cathode. Power supply 8.96V.

Anyone know what's going on here?

Thanks
dave