Mitsubishi Repair: 1987 dodge D50 2.6 engine slus in 5th gear, dodge d50, clutch components

Question
I have almaost totally rebuilt this truck trying to fix this problem and so far nothing has helped.  When shifting into 5th gear, the truck slows then takes a while to get to its fine max speed of 55mph.  I have had the engine and transmission rebuilt and put in all new clutch components and thought that the carb may be causing the problem, BUT put on a brand new weber and STILL the engine slugs when going into 5th.  Please help. I am mystified!

Answer
Daniell: How about the fuel pump? Is it putting out enough fuel? The timing, the belt or even the engine timing belt? I had a 1986 Dodge pick-up and it was a bit on the slug side when going into 5th gear. But it turned out that it has a bad leak in the rear end and so once I fixed that. It did better, but heck it has over 100,000 miles on it too. But I would have that fuel pump tested it might not be putting out as much fuel as you need when going into 5th gear. I mean you have replaced about everything else. It takes three things for a engine to zip around the road; fuel, stark and air but then the computer has to mix it all right to get things going right. How is your computer? Hope this helps and good luck! Merry Christmas too....