Invalid device signature -ATtiny85

I'm using a ZIF socket for the chips and did a continuity check with an ohmmeter to make sure each pin is working. Everything from the UNO pin to the ZIF pin works okay.
The bootloader simply tells me that it is working on a Tiny85 @8MHz, talks back and forth with it and then gives the error msg. The puzzling thing is that it worked smoothly the first three times and the UNO and that breadboard were unused while I fiddled around with the other stuff. When it didn't work on #4, I reset and reloaded and the bootloader seemed to be running okay... until it didn't.

As far as I know, the UNO is not going to change anything on the PC so those files should be okay and reloading the ISP should work. The heartbeat light on pin 9 shows that the ISP is running; just the bootloader is weird.