Steve,

If it is, as you stated, of good quality and high purity, normally too much
gold is not a problem for gold wire bonding. (However, it can be a
reliability problem over time with aluminum wire but must here assume gold
wire due to the thickness.)

The heat sink is a potential problem depending on its mass. If the stage does
not come to temperature there could be some bonding issues. Thermosonic
bonding stages are normally operated at around 150C and most bonding
schedules are target for that.

As a quick check, you may wish to see if the quality if the last bonds are
better than the first to get an indication (assuming here that the stage
finally gets to temperature) This depends also on the size of the die and the
number of wire bonds

Too much temperature causing excessive softening of the flex adhesive or
bonding adhesive and can result in acoustic dampening which could also cause
some potential problems. Here the early bonds might be better than the latter
ones.
Again the size of die and number of bonds may have a time related influence .


Just a few thoughts... Let us know what you ultimately learn.

Kind regards,
Joe