crg

Ok folks I have a 93 chevy 3/4 ton pk that has an issue the tranny shop can not seem to find. After the pk warms up usually 5-10 miles it drops out of the overdive range and into drive NO CHECK ENGINE LIGHT, the shop can not get any other code but possibly a neutral Safety Backup error, which they seem to say may be the problem. Again no other codes were throwen all temp senors and other sensors seem to check out ok. any ideas with this will the NSB switch actually cause that, it does not happen until pk warms up show took it out with test equip on it when it did this but did not registor any errors