The problem with BASIC files is the following: The PET's BASIC RAM starts at $0401 -- and that's also what the first two bytes of a PET BASIC PRG file should state. When loading such a BASIC program into a C64, it gets nevertheless loaded to $0801 (the C64's BASIC RAM) and automatically re-linked so that all "next line" pointers point to the appropriate addresses. The PET, however, seems incapable of relocating BASIC programs so that a program thus saved to tape does get loaded to $0401, but its "next line" pointers keep pointing to their C64-based $08something targets.
To load C64 BASIC files on the PET, do the following:
Type 0 REM
Load the BASIC file
Type SYS 1024 to get into the PET's monitor
Type M 0400-0410 to dump the first few bytes of BASIC memory. You should see an 01 04. Change this to 01 08 and exit the monitor. When you type LIST, you'll now see your BASIC program. The 0 REM line will be at the top of it; just type 0 and hit Return to get rid of it.