1992 Mazda B2600I 4 cylinder, fuel injected,mileage: 93,000. My truck has been well maintained. My problem started about two months ago and I have not been able to resolve it. The problem is that occasionally the engine decides to oscillate rpms when placed in gear (it is a manual transmission), and on some occasions it takes numerous start attempts to get the engine to run on its own and I have to let it warm up for fifteen minutes before being able to drive it. Once running it will vary in power when downshifting, while coming to a stop it will suddenly lurch forward. This problem is not a constant thing it just comes and goes. There is no common time of occurrence, such as only when the engine is cold or only hot etc. I have had a mechanic put the engine on an analyzer and it did not reveal any problems. I replaced the plugs, wires, fuel filter, catalytic converter, and have added slick 50-fuel treatment along with running only 93-octane gas, none of this has solved the problem. I am stumped please help with some advice. Thanks.