904 Conundrum

Those problems may seem similar but they probably are not caused by the same malfunctioning part(s). Since your first trans. lost 3rd and reverse that means the front clutch pack was burned up or blew a seal, but most likely it burned up. When that happens burned clutch material settles in the cooler (the one in the radiator that the trans. lines run too) and most likely your new trans. now has a plugged filter because of the old gunk that was in the cooler got pumped into the new trans. Drop the pan and clean it, replace the filter, adjust the bands (the rear band if out of adjustment can cause slow engagement of reverse and if the front band is out of adjustment it'll let it slip when shifting), flush out the cooler with cooler flush, install the pan and fill it with fresh fluid checking it in neutral. Probably take 4-5 qts. Test drive it. If it still doesn't shift right check the throttle pressure linkage adjustment. It should go all the way back or nearly all the way back when you floor the throttle. If it doesn't adjust it so it does. If you go too far it may want to hang in gear too long so it's a balancing act on getting it adjusted and may take 3 or 4 shots to get it dialed in. Any time you replace a trans. cause the old one was burned up YOU MUST FLUSH THE COOLER, No if's and's or but's. If you don't you will likely have problems or at the least take some life off the new trans. If you happen to use the torque converter from the trashed trans. in the new trans. it's a double whammy. Never do that unless you have it flushed and checked. Wouldn't hurt to also check the shift linkage adjustment too like 72swinger said.