Robert Frade's question about flat playing, especially in the upper register is symptomatic of not having the hand far enough in the bell.  Thus, it is the lower and middle ranges that are SHARP!

As an experiment play a comfortable mid-range note with your normal hand position.  As you play pull the hand out.  Does the pitch stay relatively the same or is there a significant difference in the intonation?  The later is what you should aim for.  As a caveat, if the hand needs to be significantly inserted into the bell, you may wish to use a more open cup to your hand to alleviate undue muffling of the tone.

Another good way of checking for correct hand position is to play a mid-register note open, then stopped.  If you have to adjust the hand position in, so be it.  Go to the position that affords you the best stopped horn sound/response.

After you have found this spot, all you need do is to unseal the bell with your palm and obtain a very flat  cup with your palm.  This should significantly improve the phenomenon you describe.