although you checked the 1, 2 and 5 rec's i would also check the 3 and 4 rec's, if there are any issues with scr speeds like you describe those rec's can be the cause of no scr variation with an audible hum present.
You should read about 7 to 12 ohms (anode to cathode) and no reading (cathode to anode).
From the sounds of it you have scr hum with little to no power with normal scr hum.
The sudden jumping when you press the pedal would be because your making the 1A switch and it is jumping into 1A speed. That sudden jump causing the huge increase of current all at once across the directional contactors, it could be a pmt trip causing the arc'ing. Then the machine stops after the pmt trip. Reset the key and it runs again doing the same thing.

While your checking components you may also want to consider replacing the 22 and 25 rec's, they can test ok with an ohm meter but still fail under load. Try swapping positions on the spider block where they are mounted or if you have another truck that runs ok just borrow those and swap them out and see if there is any change.