Alright, it all started like this: I recently bought a 91 burban 3/4 ton 4x4 with a 4L80E. This is the third trans (complete reman 20K ago). Rig has 165K on engine. When it gets up to 68 mph in OD, the torque converter goes into lockup, then it goes out of lockup like 10 seconds later. Then it repeats this many times, until I slow down fearing damage. Al the while, no codes are being thrown. Put it on the snap on scan tool. Few things that I noticed: 1: the input and output VSS vary ever so slightly 2: the TPS at no pedal didn't go to zero (tweaked it and now it does) 3: no high pressures/odd temperatures/etc 4: at idle, torque converter slip varies around (jumps from 0 to 10%) 5: The computer says I am going 9 MPH slower than my speedo (factory everything, no odd tire size change, and I have verified the accuracy of the speedo: it is dead on) Have changed the following as a result -coolant temp sensor (for $hits and giggles) -TPS -Brake light switch (often a hidden culprit) -TTech flushed the trans After these, still does it. So, I went to a trans shop that has a computer to override the ECM (He called it a Transstar?). He plugs this into the ECM plug and the trans plug, and can manually control the works. He activates the solenoids, the TCC, and the pressure solenoid. Everything electrically works fine inside the tranny. If he tells the TCC to hold lock, it holds lock. so, this means that something is telling the ECM to not maintain the TCC lock, then the ECM takes it out of lock. Then the ECM tries to do it again, so on and so forth. His feelings were this: the engine is gummed up, and it throws off everybody else. Too much carbon in the engine, cat may be bad (original cat), compression may be low (doesn't burn oil). He recommended doing a complete tune-up (was going to do anyway), get a good intake cleaning chemical (like hydro blast) to clean the intake and the valves, clean the TBI body and injectors, and start from there. Check compression ratio, etc. Next, if that doesn't do it, replace the DRAC unit. I thought that my vehicle didn't even HAVE a DRAC (input and output VSS wired direct to ECM, where trans controlled). If that doesn't do it: engine is too tired and replace. It runs rather well, and I don't accept that as an answer. It idles well, has good pull, and never makes a peep. No lifter noise, nothing. sure you can't squeal the tires, but this is a 12 year old suburban. So, I guess what I am getting to here is: -has anyone had carbon problems give them issues like this? -anyone know where the DRAC is located? -Is it worthwile to replace the output VSS? -Is there a way to check that the VSS is even bad? Any help here would be appreciated. I am getting tired of chasing phantoms here.