ICD-U64 with 18F46j50: verification fails

[kristofvl]

I'm having a strange problem reoccurring during the verification stage in programming an 18F46j50 with an ICD-U64.

My configuration:
- the CCS compiler 4.1
- the latest ccsload (4.023)
- several (5) boards, all equipped with a PIC 18F46j50
- MCLR pulled up with 10k resistor
- board powered from USB port's 5V, properly converted to 3.3V
- MCLR, PGD, PGC, GND, and VDD (3.3V from the board) connected to the ICD

The problem:
A small piece of code (turning an LED on and off) tends to work perfectly, but as soon as I go for larger program sizes (more than a 1000 instructions), I get verification errors.
The "Verify Data" phase will take very long, and end up with a "Could not read target ROM: Did not ack message"

Strangely, when afterwards checking with "Verify Chip=File", it goes faster and smoother, and returns an error on the fuses: the first byte is fine, the others are all 0xFF's.

Anyone experiencing the same?