FT-817 synthesizer problems ?

Today i had a look to the output spectrum of my FT-817 i use for 23cm operations. I looked to the IF signal of 144MHz.
There are several spurs with a distance of around 28.5kHz of each other and <50dB of the wanted signal. This corresponds to my observation that strong neighbour stations can be heard in regular steps again and again. It happens in CW and also in SSB mode. Does anyone have a idea what could be the cause ? I assume the problem is somewhere in the synthesizer... Update 2010/10/01
When looking to older screenshots i saw that they were not that spurious. The difference was, that i did not use the switched power supply from Alinco at that time but a linear one. So i checked again with a linear supply today. Result: Most of the spurs disappeared. Just some weaker ones are remaining.

FT-817 CW constant tone, 5W, span 400kHz