Hi Alan,
    As you know the Picstart is "not supposed to be used for ICSP" - however
I also use it all the time for ICSP. But however, you do have to watch the
wire lengths(a few cm at max) and the load that the "other circuitry is
presenting to the PS+ (it has zip of a drive capability). Make sure nothing
else is connected to the VDD line and try it again(I assume RB6&7 are
buffered adequately from the circuit). I've recently had a problem
programming a 628 where the code was getting in the first time but second
time it was screwed up and config bits were not being set etc (similar to
your case) - the offending item was a 100nF cap across the VDD/VSS near the
pic. Removed that, and everything worked perfectly.
You could also check that the clock hasn't started to oscillate - it
shouldn't when programming.

John
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