These reasons can include anything from a simple problem with the spark plugs, such as a worn plug or two, a loose or faulty lead, etc. This sounds silly, but it is possible, experience speaks in this case.
A faulty catalytic converter may equally have the effect of restricting speed, as may problems within the transmission system.
Correctly determining the cause of this problem can only be done by someone who knows what they are doing. Recruiting a friend or family member who has experience with car maintenance will obviously be the cheapest option.
Failing this, the car will have to be taken to a garage. Professionals there have the right equipment to quickly determine the cause and resolve the problem.
It is wise to get an estimate for the repairs first. Transmission problems in particular can prove to be costly. If there is a major problem, the car owner may also consider getting several quotes for repairs, as prices often vary significantly.
Depending on the age of the car, a major and expensive problem may also raise the question whether it is actually worth going through with repairs.
Older cars do tend to develop one problem after the other once they start. This may sound strange, but it is quite logical in a weird and wonderful, or not so wonderful in this case, way.
Replacing a major part within an old engine will naturally put additional strain on older parts trying to keep up with the efficiency of the new one.
Replacing an old car may ultimately be a more sensible and cost-effective solution.