Ah, okay, this is good information. First, what cells are you using in the Tx? If rechargables, try alkalines. If alkalines, are they brand new? If not, try some brand new ones and see if it helps. If none of these make a difference, it's time to fiddle with the POT. You can find an article on how to do this at:
http://mywebpages.comcast.net/qfm/racing/mod_04.htm#15
While you're in there, check the obvious things such as the antenna contact point on the PCB etc.