1998 Buick Riviera - Vehicle has been running smooth suddenly it starts to lose power and will not accelerate. I pull off road turn off ignition and restart vehicle and it drives fine but then loses power again and will not accelerate. Stop, start engine and it accelerate but the same thing happens over and over.

As per your problem mentioned, it indicates the problem with fuel pressure.-------- The fuel pressure is low.---------- First thing you can do is get the fuel pressure checked, this will confirm the problem. --------- If the pressure checked low that means problem is with faulty pump and clogged fuel filter. -------- But if pressure checked ok,then in that case, other possibilities can be problem with o2 sensors,clogged converter,worn out injectors, problem with harmonic balancer, faulty crankshaft sensor.------------ The o2 sensors are before and after catalytic converter.-------Both the sensors are to be replaced.------ And as per you mentioned your problem, its quite possible that faulty o2 sensor will cause this problem.------ O2 sensor is the part which sends signal to ECM( the cars computer).It sends signal to computer about fuel / air mixture.How and when to send fuel to engine.--------- If o2 sends stops sending signal to cars computer then it will not send the fuel to engine.------- If the fuel pressure is checked ok, then its quite possible that the o2 sensor is getting overheated and this is stopping the o2 sensor from sending signal to ECM.But when your car shuts off the sensor gets cool and again it starts working.------ So as per problem, get both the o2 sensors checked and replaced.---------- Also get the crank sensor checked.---------- The fuel pressure should be 41-47psi.-------- But if fuel pressure o2 sensor, crank shaft sensor, injectors all checked ok, then, the harmonic balancer can be the issue.------ It may have jumped a tooth on the timing chain. Hope not. With the ignition off, put a wrench or ratchet w/socket on the large bolt in the center of the harmonic balancer and turn the engine clockwise, just a bit. Then, slowly turn it the opposite way, noticing how much 'slop' before it becomes hard to turn again.
Anything over 6 degrees (1/4") on the outside of the balancer is excessive. If this has actual timing marks, you can start at zero and take an accurate measurement of the degrees. ----------- The role of the harmonic balancer is to absorb energy and reduce vibrations in the crankshaft. It is an important component because the vibrations can be strong enough to damage the engine, as well as the accessories driven by it. The balancer is a composite device made up of metal and rubber parts. Rubber is prone to deterioration and this can cause slipping or even complete failure. Changing a balancer is a difficult task that should be left to a professional mechanic but it can also be done at home. Special tools are available that can help with this task, such as the harmonic balancer puller.-------- The harmonic balancer is located at the lower front of the engine. It is attached to the free end of the crankshaft, opposite the end that is joined to the clutch and transmission. The balancer is comprised of a metal inner hub and outer disc, with a thick rubber layer between them. The size and weight of the parts are designed to absorb specific vibration frequencies. This means that a balancer should only be used on an engine it was designed for, otherwise it will not be as effective. Most balancers will either have groves or a separate pulley attached to them for driving the accessory belts.--------

As with all engine components, the harmonic balancer will eventually fail and need to be replaced. The rubber layer between the metal parts is often the source of the failure. The adhesive bond between them can fail and cause the parts to slip, reducing the amount of power transferred. The engine accessories may not function properly when this happens as they do not turn at the required speed. The rubber can also break apart internally and cause a total failure of the balancer. Cracks on the visible edge of the rubber layer are a sign that it is deteriorating. The seal behind the balancer should also be replaced if there is oil leaking from it, as oil makes the rubber deteriorate faster.

It is often difficult to remove the harmonic balancer because of its tight fit with the crankshaft. However, a special tool called a balancer puller can make the task easier. The puller is a metal hub that attaches to the balancer with several bolts, and has a center bolt that presses against the end of the crankshaft. When the center bolt is turned, the puller pushes against the crankshaft and draws the balancer off it. After the balancer has been removed, the shaft key and slot should be inspected for damage and the key replaced if there are any chips or cracks.

Installing a harmonic balancer can be just as difficult as removing it. The contact surfaces on the crankshaft and balancer should be lightly oiled so that they slide together more easily. The balancer can be pushed onto the crankshaft by turning a bolt in the center hole. Since the locking bolt is too small for this, a larger bolt must be found with the same thread. To avoid stripping the thread in the crankshaft, the bolt should have several turns on it before any force is applied. It may be necessary to first tap the balancer on with a rubber mallet. Once the balancer is on the crankshaft, the locking bolt is secured with the correct torque.

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Start with fuel pressure checking.If fuel pressure checks ok, then go for other troubleshooting possibilities.--------

This will help.Thanks.Helpmech.