Intermittent problem. 2007 G6 base model. 4 cyl. 150,000 miles. Happened 4 or 5 months ago. Car lurched while driving 70mph, tach dropped to 0. Mechanic couldn't make it happen the next day. Happens more and more now. Has started chugging while going uphill like it isn't getting enough gas. Seems to be shifting hard with tach problem. Shifts fine otherwise. No warning lights. Please help!