this is for my dads truck a stock 99 with auto trany, the problem I'm having is after truck gets to operating temp you can be driving along at say 35-50 and it feels like truck goes to neutral, it slows right down and you can press the throttle and it just revs like it is in neutral.but when I hit the od switch and light comes on dash it goes back into gear and drives fine until i hit the switch and light goes off again it will shift normal and then a few seconds to quite a few seconds later it will act up again. it does this quite a bit but only at operating temp. shows no codes on my scanner any help appreciated. thanks

 

Sounds like dad needs to spring for a tranny overhaul. Drop pan and have a lookee see. Probablly find clutch material in pan and magnet oversized.

 

pan was clean (no clutch material) and magnet had the normal amount of iron on it(just enough to lightly coat magnet)

 

Hunmn!! In that case, the rpm sensor on left rear of tranny could cause drop out if signal went away at 40 MPH. Looks like would do at higher speeds. For the $20 sensor, just replace instead of trying to diagnois. If the brake switch comes on at those speeds it will unlock also, but it doesn't go to neutral.
Read the TC lockup thread at top of this forum. Some good stuff there.
It sounds like the front clutch is releasing which would cause neutral feel. Have someone with code reader look for any codes.
Recheck all grounds and make sure both batteries are good.
Previous statement about going into neutral convinced that front clutch was out. It could still be a erratic seal. There are 6 seals involved in that circuit.