700r4 converter not locking

[Mac]

After reading Gilles' transmission saga and Sum's tecnique for T-shooting a torque converter for lock-up I thought I'd try it on my `92 Chevy K1500 daily. I've been suspicious about the `verter's lock up lately and sure enough I get RPM flair when I give it some throttle crusin in 4th at 50 on level.

Question is: Is the problem likely to be switch, line pressure (or lack), the `verter itself or what? It's hooked behind a stock 350tbi and the truck's got 120k on it.

I'd apreciate any help diagnosing this. TIA

Jack