At 40 I think there's just enough pressure there to lock it up. The light throttle adds just enough pressure to it to have it unlock. At that speed the converter gets "confused" of what to do. That's why it's locking and unlocking. Even when my truck was stock, a long time ago, If I was going under 45 I'd lock the o/d out. If I was going a steady 50 or more, then I'd give it o/d. 40 and 45 mph are hard speeds on a torque converter and trans. People just don't realize it. The way my truck is now, my t/c will lock up in third and stay locked up even as low as 20mph. If I'm slowly gaining speed the t/c will lock at 45mph in o/d and stay locked until it gets down to about 42.
Different issue, I recently had my t/c lock and unlock even at highway speeds. People have wrapped some wiring in aluminum foil, added extra grounding off the tps, disconnect BOTH batteries and short the positive and negative cables together overnite, etc/ to try to resolve this issue. I disconnected the plug at the alternator and drove it for a couple of days. It has not done it yet. Key word in that sentence, "yet"!!
99 2500 q/c 4x4, 4" Silverline exhaust, Edge Comp Hot Unlock, DDP 125hp sticks, BD Tow Twins/intercooler, FASS 150/150, Suncoast triple disc tc & vb, Mag-Hytec dbl deep pan, ARP's, fireringed, SnowPerf W/M, A-Meter Ultra Lite pyro, boost, trans, fuel gauges.