Stump the chumps:  I've got an 85 auto with a rebuilt engine with
about 5000 miles on it.  It's running ok overall (mpg around 17-18 in
fast california driving), but power seems a bit off (even for vanagon
standards).

The problem:  The engine makes a ticktick sound at around 3000 rpm
that was definitely not there in the prior engine.

It sounds a bit like ping, but the behavior is not always ping-like:
  * only happens under 1/2 to full throttle under load
    (consistent with ping)
  * it's worse at altitude and cold
    (backwards? ping is usually worse at sea level on hot days, right?)
  * not obviously connected to the octane of fuel.
    (inconsistent with ping)
  * the frequency of the tick noise sounds like it may be happening on
1 cylinder only
    (e.g. unlike ping, it doesn't seem to decrease in frequency when
you add more
     throttle, just the loudness increases.  When i've driven cars
with ping,
     usually the ping frequency is dependent on throttle/load too)

It has rebuilt injectors that were flow tested at witchhunter.  The
valves have never been adjusted, and I *think* the shop that installed
the engine (to which i'm unable/unwilling to go back to them...a long
story) may have replaced the distributor and messed with the mixture
adjustment screw, as well as putting in a new (rebuilt) ECU.  I'm
running mobil 1 15/50 w/mahle filter.

Ideas?  I'm having a real mechanic look at it, and would like to make
sensible suggestions...