1991 Mazda Miata wont shift out of 1st gear

Have an automatic 91 mazda miata with 120K. Now the car wont shift out of 1st gear. Checked the ATF and its really dark and saw some metal shavings at the end of the dipstick. My girlfriend said she might have heard a knock around the time it stopped shifting, but not for sure, and she said it chugged a bit just before taking off in 1st. Although, she said that when she tried to back out of the parking lot to leave it would roll back like 3 or 4 feet, but thats it. Now the only gear that is operating is 1st. No reverse either. I talked to a guy that deals with miata's, and he said that he had the same problem with one of his vehichles. He said that he had an transmission flush, and it solved the problem. Is this possible? Before this all happened, for about a year, the transmission had a slight problem getting out of first. If I eased on the gas in 1st, then it seemed to shift to 2nd OK. But if I accelerated quicker, it seemed to shift only after revving it up really high. The same guy told me that it was some kind of switch that you needed to get into the transmission to fix. Wandering if you think I should try the ATF flush or just go with a new tranny, and also if you know anything about the late 1st gear shifting. Cant find anything on either matter. Thanks