On our way to football on Saturday, started the car and the battery light stayed on for a short time, as I was turning round the car seemed to rev a little. Had to stop on the way but left the car running, got back in the car selected drive and nothing, went through the gears and still nothing, then got the dreaded TRANS FAIL SAFE WARNING. Switched the car of started it up, hey presto we have gears, drove a mile or so and the car started revving and ground to a halt, switched it of started it up and away we went and so this continued for about 4 miles until we got home. The cars now sitting outside and wont go into gear. Expecting the worst when it goes in to the workshop  2004 vogue
1978 Pontiac Firebird
2010 Fiesta

There is a lot to read up on here regarding the TD6 GM gearbox. I don't have that type myself, but the first thing you would need to be sure of is fluid level. You might want to speak with a gearbox reconditioners, and have it looked/diagnosed...rather than a general mechanic.

You mentioned the battery light went on. The FFRRs do many weird things if the power isn't up to snuff. Battery and alternator are always great places to start you troubleshooting. Good luck.

My mate who has his own workshop popped round yesterday with his diagnostic equipment, it all points to the torque converter. The fattys getting recovered and of to the workshop in a couple of days. 2004 vogue
1978 Pontiac Firebird
2010 Fiesta