He got it back immediately with a birdie at the long 15th, added another at the long 17th and just missed his birdie chance from 12 feet at the last.
Frost made three birdies in a row from the seventh but then made three bogies came on the 10th, 14th and the 18th, with two other birdies in between.
Three players are in joint fourth place, six shots back — home favorite Sandy Lyle (69), Australian Peter Fowler (70) and first-round leader Gene Sauers of the U.S (70).
Colin Montgomerie of Scotland also had a 69 but is tied for 13th at 2 over. Miguel Angel Martin of Spain had the day’s best round of 65 but is also on 2 over.
Defending champion Fred Couples had a 68, his best round of the week, but his out of contention at 4 over.
Ireland’s Mark McNulty shared fourth place overnight but quickly fell back when he dropped a stroke at each of the first four holes and another two at No. 7 and 8. He finished with a 76.