At times driving between 35-40 mph in overdrive the tach will bounce up and down 2-300 rpms (1200-1500 rpms) but not all the time. The engine speed does not seem to be surging. It is a 3500 engine with 14K miles. The tach is steady at 60 mph at 1500 rpms.

My parents 2007 Impala does not do this and on a business trip recently I drove an '09 Impala that did not do this.

All cars have the 3500 engine. Is this normal? The dealer looked at it once and saw nothing, but that (the selling) dealer has closed and my warranty will be up soon in July.

Sometimes slowing down seems to "grab" coming out over overdrive into 1st gear as well...

Thanks in advance.