85k miles, intermittent issue, but more frequent all the time. In the 1500-2000 rpm range in Drive or 3rd, car shudders & rpm swings +/- 300 or so. Will not do so in 1st or 2nd gear in same or any other rpm range, so I’m thinking that it is not an engine issue. Does the converter go into lock-up in 3rd gear, or only Drive? If I drive it HARD it seems I can get it to come out of it for the most part. Any ideas?
If your gear selector is in drive range then the converter clutch will lock up in 3rd gear. If you are driving with the gear selector in O/D range then the converter clutch will lock up when the transmission reaches 4th gear. If the trans is falling in and out of lockup the first thing a shop needs to do is to connect a scanner and monitor TCC live data. The scanner can tell the trans tech whether this condition is slippage or not and whether the condition is computer commanded or not. If its slipping excessively the scanner will tell you a percentage of slip or the slip rpm’s. The scanner will also tell you whether the converter clutch is locking and unlocking electronically. If its electronic then the tech needs to check for the source or find out what exactly the computer is seeing thats telling it to lock and unlock the converter clutch.
transman
Thanks Transman. I tried to find a tranny shop open this morning, no luck, I guess business is good! 
This is my kids’ car, so I’ve only driven it a bit, but this morning I drove it for a fair spell, and the problem does go away when driven a 1/2 hour or so. What I noticed was that initially the frequency of the surge was fairly well spaced, becoming more and more frequent as things warmed up, it really hammers, not any harder, but the lock-unlock gets closer and closer together until finally it works properly. Wondering if a trans fluid & filter change worth a shot, this was done @49k, now have 85k, wouldn’t think necessary, but… So does converter lockup occur based on a buildup of pressure or a drop in pressure? Warm fluid would be lower in pressure, unless of course the filter is dirty and warm fluid passes through it more readily. hmmmm…