Your diagram pretty much matches what I would expect to see for a compressor installation.
I redrew your sketch to show things a little more clearly.
I can think of three possibilities as to why it seems to be erratic. The pressure switch contacts could be going bad which I don't think is the case since the pressure switch is not wired on the high current side feeding the compressor motor. It's also possible the pressure switch is just failing inside and not registering pressure properly.
Second, The circuit breaker (You list it as a relay) could be going bad and third, the solenoid is going bad. Or one other possibility is a loose wire somewhere.
Based on your symptoms and your wiring my diagram I would consider the solenoid to be the most likely culprit.
The reason you were getting a 1 amp draw previously is the pressure switch was closed calling for the compressor to start which energized the solenoid. However with the toggle switch in the off position the compressor can't run. Whoever installed the toggle switch apparently wanted to make sure the compressor wouldn't run unless they wanted it to but they failed to take into account the solenoid would still be energized if the pressure switch was closed.
An easy fix would be to move the toggle switch to the left side of the pressure switch which is where it should be anyway and run the ignition hot signal from the fuse block directly to the pressure switch.
Also, the white wire that comes from the pressure transducer should head to the dash for an air pressure gauge. The white wire likely will not have a voltage reading or if it does it will be a low reading.