Transmission

1994 Buick LeSabre
automatic
65,000 miles
not sure of engine size

The car starts fine and runs I presume in 1st gear fine, but then when I get to about 10-15 mph, it revs very high. I don't have a tachometer but you can hear it cranking. This is a new problem, so haven't driven it much, but it did seem to clear up when I got up to about 35-45 mph, so I think something is wrong with 2nd gear. Check trans fluid and its full, but don't remember when last had it flushed.

Any idea what might be wrong, and roughly how much that will cost to fix at a garage? Thanks.