I too have that issue with my gauges. It happens when there are drastic temperature changes inside or outside of the instruments. I believe this is because they're not sealed properly.
I recommend exchanging them for some nice VDO Vision or Industrial Black guages. They're extremely similar to the stock dials if you want to retain that fashion. Plus, the illumination in each dial is far superior to that of the stock.
If you're interested I'd go to www.egauges.com
Their prices are reasonable and they supply other series of VDO and other makes. If you want to do the whole dash I priced out the following to provide you with an idea what it'll cost:
All are Electrical
Temp Gauge Vision 2 1/16"
Fuel Gauge Vision 2 1/16"
Tack 6K RPM Vision 2 1/16"
120MPH Spdmtr Vision 3 3/8" (Not sure if this it the correct size)
Total with shipping: $270 Which isn't too bad considering you're stock speedometer is probably more than $300. You can modify some of the dials to narrow or expand their readouts too. I'm sure a couple of guys on this board might chime in with their experience with it. I know a few have built their D90s from scratch and have elected to use other gauges. Hope this helps.