I've used all kinds of filled Teflons, G10 and other epoxy mixes, nylon, some others.  It all depends on what kinds of stresses and impacts the material is expected to see, and what the material is to be used for.  If you can provide more info on the application, and what stress levels (if structural) it would help to reply to this Q.

Have you thought about using a high temperature Polyimide?

I tried Vespel SP (think it was SP21) for piston rings and rider bands on cryogenic pumps which failed misserably.  Also tried Carbon fiber filled PEEK for a valve part which failed misserably.  

It's very important to understand the impact loading, and other characteristics of the plastic and how the plastic you plan on using will handle them.  So all uses of Vespel in cryogenic service may not be bad for example, but there are plenty of other materials I've found which are suitable and better for the vast majority of services I've had.  

I have had success with Torlon 4301 in a valve sealing application.  It is a compressive and impact load, cycling many million times before failure.  Kel-F is used frequently in valves and has good sealing qualities.

Vespel can also succesfully be applied at low temps. When mech. strength cq accuracy is of high importance and (higher)conduction of lower importance, why don't you think of stainless steel (AISI 304 or 316).

Thanks to everyone that has responded to this post. All suggestions have been most helpful. We actually found that polycarbonite can withstand our temps very well without a severe amount of thermal expansion or in this case thermal contraction.

Thanks to all,
CADShark