You most likely have one of two problems: a fuel line restriction or an exhaust restriction.
A plugged fuel filter, crushed fuel line or clogged pickup screen inside the fuel tank can all starve your engine for fuel. Enough fuel may get through for the engine to start and run at low speed, but when more fuel is needed it can't get through resulting in loss of power at high speed. The same kind of problem can also be caused by a weak fuel pump. But fuel pumps usually quit altogether when they fail.
Inspect the fuel line from the tank to the engine. If you don't see any obvious damage, try replacing the fuel filter. If that doesn't help, blowing out the fuel line with compressed air from the engine towards the fuel tank may help dislodge a blockage and debris from the pickup screen. If the pickup screen in the tank is clogged with rust and debris, the tank will have to be removed so the screen can be replaced and the tank cleaned.
Fuel pressure can also be checked by teeing a suitable gauge into the fuel line. If pressure is very low (refer to a manual for the specs), the pump probably will need to be replaced.
As for an exhaust restriction, the easiest way to check for this condition is to hook up a vacuum gauge to a vacuum port on the intake manifold or throttle body. If vacuum is low and continues to drop as the engine runs, it's telling you pressure is backing up because of a restriction in the exhaust. The most likely culprit is a clogged catalytic converter.
If the converter has overheated, it may have melted internally. This would restrict the flow of gases through the converter and create a serious backpressure problem that would cause a loss of power at high speed. If the blockage is complete, the engine may start then die and not run at all.
Other causes here may include a crushed exhaust pipe (a visual inspection should find this easy enough), a double-walled pipe that has collapsed internally, or a muffler that has become clogged with rust (rare, because they usually blow out). If you suspect an exhaust restriction, temporarily disconnect the head pipe from the catalytic converter (which is no easy task because the bolts will probably be rusted solid). If the engine now runs normally, you have an exhaust restriction. Inspect the converter and replace it if needed.
NOTE: If the converter is plugged, it failed because something else caused it to overheat. Causes include misfiring spark plugs and leaky exhaust valves. The underlying problem needs to be identified and corrected before the converter is replaced otherwise the new converter will suffer the same fate.