Has the service provider fixed the problem, but you fear further irregularities? You can send the service provider a warning letter and you reserve the right to terminate the contract without notice (also called extraordinary termination) in case of further problems.
Termination without notice in case of disruptions
You can terminate your contract without notice only if you have an important reason. It entirely depends on the individual situation. For example, the disruptions or failures must be permanent and you have to prove them (for example by logging the poor Internet speed). In addition, the failure must be within the risk area of the service provider. For example, if the connection is interrupted due to a storm, then this is not within the risk area of the service provider and you cannot terminate your contract without notice.