First, welcome to the site.
If your
tranny is slipping in 3rd and 4th, then it needs to be serviced, or even rebuilt. You can get some better upgraded clutches and bands, larger drums to hold bigger clutch packs, and even a gearing upgrade that has more torque holding capacity. A shift kit or aftermarket valvebody would be a recommended upgrade and maybe a deep pan to give more fluid capacity. You could get a torque converter with about a 2000-2200 stall speed and that would make a world if difference, but there's two kinds. Lock up and non lock up, the lock up locks out the converter in 3rd and 4th, where the non lock up doesn't and it multiplies the engine torque in 3rd and 4th. The non lock up is stronger than the lock up because it uses a 1 piece input shaft, the lock up uses 2 thin shafts. Non lock up takes a slight amount of your gas mileage away but not enough to really worry about. Hope this helps.
New member with stock 1988 LX 5.0 convertible 
