some times ago, someone said you must choose oscillator HS (high speed) for the programmation to work on these chips. if you choose XT, it fails...
give it a try !
i would advice choosing also a different oscillator
the other thing is you may need to shorted the lead or add some screening
perhaps as it is intermittent
you havent been carefull enough to set the voltages
try to set up the unit again
making sure that a good ground between the pc case
try three things
1} enable spp standard port in bios
2} use a short cable lengh
3} use a 17V ac supply {at least } 18 is best
4} choose a 2200 uf cap as the main resoviour and 220 as the cap that smooths the vcc supply from the 8v regulator
5} mount the programer on a bit of single sided pcb not etched
then ground this to the gnd rail on the programmer
to add screening so mount it less than 20 mm from the bottom with the pcb foil under so it cant ever short
add some stand offs so you can push it in safe {adds stability}
6} dont twist up cables and keep it away from monitors and other emf
7} plug the transformer for the unit into a socket bar with a filter on it
perhaps your mains is dirty
the fact that it programs sometimes and not others is a sign of electrical problems
find by writing down what settings works "sometimes" and in adding these tweaks youll get a nice working unit