IF this is an ev1 system
as soon as you depress the accelerator pedal and the directional contactor comes in you should hear an audible hum comming from the control panel.
As you push the pedal down further that hum will speed up
Are you hearing this?
If not then you are on the right track with an scr issue.

Also if you are having to push the pedal down further to get any reaction out of the system it may be that you are pushing it down to the point that it hits the 1A switch in the accelerator box and this is why it is jumping 2ft....
WHEN this happens if it tries to go into 1A speed and if there is a problem in the SCR range the PMT will see this and trip the truck out.
This is why you have to shut the truck down and start it back up. Cycling the key resets the PMT circuit.

Being that you said you checked the scr's and found some problems with a couple of them that is more than likely the issue.

Get the parts and install them and see what happens :o)