I2C bad clock stops transmission

I'm using ADuC7026's I2C to interface with AD5254 (digital potentiometer). Every once in a while, I get a bad pulse on the SCL line, and the AD5254 does not acknowledge the byte. The bad pulse is essentially a pulse that only rises 40-50% of the normal logic level (3.3V). Subsequently, the SDA line remains low, and I must do a hard reset of the micro to get the line back to it's high logic level.

This condition seems to occur more frequently when I'm interfacing with a host PC through the ADuC7026's serial port.

Any help would be appreciated.
Matt.