Did the car come with the FTC warranty window decal? Most dealer re-sells will and are required to conspicuously post whether a car is sold as-is or not.
"AS IS - NO WARRANTY"- If this box is checked this means that you are buying that used car with NO express or implied warranty. This means, simply put, that if your used car suffers a major breakdown on the way home from the lot you are stuck with paying for all the repairs yourself.
"WARRANTY"- If this box is checked some sort of warranty/service contract is being sold with the vehicle. The FTC Buyer's Guide, as a practical matter, prevents the disputes that used to occur regarding whether a used car was sold with an IMPLIED warranty. CAUTION: Don't assume that you are buying a used car in excellent condition just because the warranty box is checked. The fact that a warranty or service contract is offered along with the vehicle is NOT an assurance that the used car you are about to buy is in good condition. Only a good auto mechanic can tell you that.
REMAINDER OF MANUFACTURERS WARRANTY: If the used car you are buying still has a manufacturers warranty that has yet to expire you are protected under the "Lemon Law" as if you purchased the car new and can assert your rights against the manufacturer in the event the vehicle is a "lemon".
WARRANTY PROVIDED BY USED CAR DEALER: Most used cars are sold without a warranty. As a practical matter most used car lots lack the resources to give buyers a warranty themselves. Used car dealers normally sell service contracts handled by an independent company. However, used car dealers will sometimes give a brief written warranty (i.e. 30 days) on a few used cars they sell. The dealer then becomes as responsible as the manufacturer in the event you have a "lemon".
You also receive, as a matter of law in California, an implied warranty of merchantability (and possibly an implied warranty of fitness) between 30 and 90 days. I would mention that and demand that they fix the car. If they give you any probelms, talk with an attorney.
More facts would be helpul, including the nature of the conversations at the time you were induced to make your purchase.
You are not without recourse.
When a seller is a merchant, the law extends an implied warranty that a product is fit for the ordinary purposes for which it was intended. This is called an implied warranty of merchantability. This gets tricky with used cars, and it would be helpful to see all of the paperwork you received.
If they are honoring your warranty and being cool about the repairs, try to work with them. The reality is that with nearly 100K miles it would not be reasonable to expect new car performance.