Toyota hilux 1993 model 4x4 running hoping and stoping on 100km/h and if you drive on 60-90 km/h is running smooch.what can the cause or problem be.

I am going to guess you have the 2.4 diesel. Water in the fuel system is deadly to a diesel. Make doubly sure there is no water in the fuel filter. Most have a drain in the bottom, some have a water detector. If you do not have these features. I would replace the fuel filter. How did water get into the tank? The entry of water needs to be stopped. Air in the Injection system is a major contributor to poor performance. It's likely that, with all the work you did that air is in the system. If the fuel in the tank is sucking air, or a pinhole somewhere along the supply line to the filter may allow air to get into the system. Usually, the return line to the tank is clear in the engine bay and it's easy to see if air is going through the injection pump and back to the tank. The manual priming pump will also demonstrate this. Push the priming pump to see if you get air bubbles streaming back through the clear return line. The line is too old to see? Replace with clear so you can see what's happening. I had a problem of air getting sucked through an electrical connection on the top of the fuel filter mounting bracket.
A different diesel, but most filter arrangements are similar. I was able to seal the air leak with an oil-fuel resistant sealer almost liquid so that it could be "sucked" into the leak. The similar highly liquid windshield silicone leak sealer may be a good sealer.

Did you replace the fuel filter? Sounds like it is clogged.