Welcome to JustAnswer, I'm Neal and I'm here to help get your problem resolved. Please feel free to ask follow up questions, it is part of the process.
Based on the info provided I would say it is the plug wires or one or more of the ignition coils.
Some parts stores can test coils but maybe not for a LR. Sometimes coils will fade out as the demand increases. Can your hear any arching from the plug wires?