I previously outlined problem - system checked, new actuator installed, but still sporatic. 1985 300 SD

Have installed two used amps since with zero affect.
With old amp plugged in, I got it to hold a couple of times, but not in the last several days. I have, however, been able to get it to accelerate using the stalk, but it will not hold when I let go.
Is it possibly a bad stalk switch? If so, is there a way to check before I go for a new or rebuilt amp?
Thanks