[Tim240Z]

I have a 71 Cheyenne 20 that I transplanted in a 99 Suburban L31 Vortec 5.7 and 4L60E. I spliced in the 99 Burb harness to the 71 with FSM in hand.
The Burb had ~62K miles on it when I got the drivetrain out of it and I have 700 miles on the conversion now. Here is what has started happening:
When on the freeway, as I reach 60mph and higher, the truck will surge, like it is hunting for a gear, or shifting in and out of 4th/3rd, or the TCC is locking and unlocking. If I drop below 60mph, it is OK.
If I manually shift it down to 3rd, the tranny whines really badly.
The only DTC I got was DTC0719 which is short in the brake switch power to the TCC. I found a loose connection and repaired it, but the FSM says the VCM ignores that signal anyways when that DTC is thrown.
Is this something common with the 4L60E at this kinda mileage?
Is it a full rebuild candidate, just TC replacement, or did I mess up something in the wiring during the conversion? Just driving around town, everything seems fine....just freeway driving. I would assume that if there was a wiring problem then a code would throw, or the tranny wouldn't shift or something?
I just don't want to replace/rebuild the tranny only to have the same thing happen down the road.....

Any help would be appreciated.....I'm great with engines (built plenty of them), but am transmission retarded.....

Tim

Before:


After:




Painted it in the driveway...not too shabby: