"avrdude: verification error; content mismatch" when programming ATTiny with ArduinoISP

Hello,

I am trying to upload a sketch from the Arduino IDE to my ATTiny boards. I find that when I upload a sketch immediately after burning the bootloader, everything works fine! However, when I try to upload a new sketch without burning the bootloader in between, I get an error that looks like this:

avrdude: verifying ...
avrdude: verification error, first mismatch at byte 0x0002
         0x30 != 0x3d
avrdude: verification error; content mismatch

(the byte index and different byte values change depending on what sketch I am uploading, but it's always the same kind of error)

I am using an Arduino Uno with the ArduinoISP sketch loaded as my programmer. I have an SOIC clip to program the boards. I am using ATTinyCore to choose the boards from the IDE. I have had the same problem with both an ATTiny85 and ATTiny841. I have a capacitor across GND and RESET, which I've heard was a problem for some. Since the programming works sometimes, I'm pretty sure things are wired up correctly. I also thought it might be due to how the ATTiny was wired on my board, but the same thing happens when I try to program an ATTiny that's not connected to anything.

Has anyone heard of a problem like this or know anything I could try?