Locking and unlocking? With TH350? That's a 700R4 thing.
TH350 is on all the times. The convertor is not locking and unlocking. I'm thinking that's bad clutch you're feeling. Slip-grab-slip-grab
Like electrical, trans troubles are hard to diagnose online.
Im leaning towards the convertor has cashed out. More than likey pump is going out too thanks to convertor chunks.
If your seeing metal in the pan and with stalling out, convertor is finished. There is no clutch left in convertor so its direct to engine which causes the stalling. Convertor needs to slip at a stop or engine stops.
Im no trans rebuild expert but been down that road more than once.
Best GM trans guy. Hands down
http://www.phoenixtrans.com