af13d should be a 2003, sg15 CARB system.
a few possibilities, any side way free-play in the distributor shaft? at all? not up and down, but side to side?
how long is the vacuum hose that runs to the ECM from the throttle body? (NEEDS to be at least 29 inches maybe as long as 34'), which looks way too long, but unless the ECM has been replaced with the newer version, this hose is "tuned" to a vacuum loss due to it's length. the newer version does not use the vac hose, and the plug for the hose is capped off inside the ECM)
Next would be to insure the vac hoses on the throttle body are in the correct places, with the hose that runs to the Dist. for vac advance is in the top and the one to the ECM is in the lower (more towards the manifold) placement.
next, I would put a timing light on the thing, and see if the timing is advancing as it should when you increase RPMs.