[THEFERMANATOR]

Does it only do it in lockup, or does it do it all the time at those speeds? If it does it only while in lockup, then my next question is did you upgrade the valve body when you did the converter? 4L60E's from 95+ sufferred from lockup bore wear in teh valve body that causes them to slip or drop out of lock-up many times throwing a P1870 code. The most common aftermarket fix is to replace the PWM lockup valve with an on off style valve to eliminate the pulse width modulation lockup function(soft lockup feature), and makes it like another gear. GM and SONNAX use an oversized lockup valve in the valve body, but they suffer from the same issue down the road. Another VERY common issue that causes missing going down the road at cruise is the injector pod assembly. If it is the origanal non multi port updated set-up this could be your issue. Also make sure your cats aren't plugged, the EGR is working and not hanging open, and O2 sensors are NOTORIOUS for giving driveability issues in these without coding or for throwing misfire codes. Also make sure and put a PREMIUM quality cap and rotor in it as the crab caps are BAD about cross firing internally.

You need to narrow it down to wether it is a driveline binding issue, lockup kicking out, engine misfire, or loading up. Then go from there. And yes, I've worked on a FEW of these animals and the early V8 VORTEC can give you nightmares trying to chase issues in them as the OBD2 in them SUCKS.