2009 Chevy Cobalt stalling

My fiancés car has been stalling, but not in a normal
way. The other day we were driving and as he made
a left hand turn the car stalled out and the only
reason we were moving was from momentum. The
speedometer said the car was going 90-100mph
but we were maybe going 25-35 (again strictly from
momentum) and the rpms were between 4-5 but
the car would not move. When he would step on
the gas the engine would rev but again would not
actually speed up and go anywhere. We pulled over
and turned off the car and restarted it and it was
fine again. This is the second time this has
happened and the only common factor is he was
making a left either around a curve or a left turn
onto another road. We initially thought it could be
due to temperatures but the first time about 3
weeks ago it was 9 degrees and yesterday it was 58
degrees outside. The check engine light never came
on in either instance. Any ideas on what it could
potentially be?