I have programmed this chip for years using 12 foot DB25 extender cables and NO buffers. Make sure you have 100-150 ohm resistors on all 3 SPI pins at the 8052 side of the cable.

Also, once you get an error using SPI ISP, it's safer to cut power completely (remove SPI cable too) to get back to a known state.

Holding the device in reset on power up, and/or putting the /EA pin in the external code mode can also help ISP if your program is causing problems with the SPI pins.