my 2007 element(bought used recently) has a SHUDDER or shake when the car is going in the speed range of 25 to 30 mph and the tack is always in the 1800 rpm range. after it shifts down, the shake is gone. our local transmission mechanic did a scan and found no codes for it. the local dealer I bought it from is trying to put me off so that my warranty runs out. any thoughts?