I rebuilt the 4L60E tranny in my 96 suburban about 1 1/2 years ago. Followed the ATSG Manual using a quality rebuild kit. Made a few performance mods. Shift kit, New Converter, etc. "Seemed" to run fine for a year or so, although I never pushed it very hard. Currently it shifts 1st, 2nd, and reverse but sometimes ( more often than not ) when it shifts to 3rd there is nothing there. as if it goes into neutral. The truck has 4.88 gears and 37" tires. The speedometer works but is not calibrated for the gear/tire mods. Also the gear selection indicator lights don't always work or don't work correctly. Does this look like a minor electrical/servo/calibration problem or is it more likely a major internal issue?