Hi. 1990 Buick Riviera, 207,000 miles, 3.8L. Automatic. When in the overdrive gear, after the engine warms up to a certain point, the car feels as though it simply drops out of gear when I'm at a stoplight or stop sign. It takes about 10-15 seconds, then pops back into gear. It does not do this in regular " drive, " and there are no other transmission issues that I'm aware of. Any thoughts on what might be causing this? Thanks in advance.