"Surging" in gear with brakes on at stoplight...

Both of the above are probably right, but timing can have a significant effect too.
Put a timing light on it, in gear. Does the timing move around at idle in gear?
If so, figure out why. Advance springs, vacuum advance, whatever the issue may be. Idle timing shouldn't fluctuate much while in gear.
Obviously block the tires or use a helper or ensure the parking brake works really good if you're going to check anything in gear..