2001 Chevy Tahoe temp gage go above normal in traffic

my 2001 tahoe temp gage runs just below 210 deg. normally. lately when in slow, stop and go traffic my temp gage goes up around 230-235 deg. if i turn the a/c off and drive a couple of blocks it returns to normal. revving the engine up also returns it to normal. does not go up on the open highway. engine coolant level ok, radiator flushed at about 70,000 mi. engine fan seems to be working ok. also if i turn the a/c off and turn the heater on the temp drops back almost instantly.
appreciate any thoughts on this.