Bascom and AVR, controlling the LT6904 programmable oscillator

The Linear Technology LT6904 is a programmable oscillator chip. Output can be set from 1kHz up to 64MHz. I wanted to test this oscillator to see if it was suitable in an antenna analyser. It appeared that the harmonics in the output (it has square wave output) made it difficult to use. It is however quite handy as a programmable oscillator to replace crystal oscillators etc.
I have made a simple test setup with an ATMega in control:

The LT6904 has an I2C interface (the LT6903 uses SPI). Also attached to the I2C lines in the schematic is an ON-Semi JCL1562 which is not used in this test.
The program cycles the LT6904 continuously between 5 and 10MHz in steps of 10kHz. Make sure you read the datasheet carefully to understand the programming.

I have noticed that with a 5V supply the LT6904 shuts down every few seconds. This does not happen with a 3.3V supply. Does the chip overheat with 5V? The datasheet states that Vcc up to 5.5V can be used. With 3.3V the chip performs troublefree.