This study examined the impact of learner proficiency on the occurrence and resolution of language-related episodes (LREs) in rehearsal of interactive speaking tasks and on the subsequent use of language items from LREs during performance of the same tasks in front of the class. Forty-eight learners from six intact English as a foreign language (EFL) classes at a Vietnamese high school were divided into three proficiency groups, each consisting of eight dyads. Group 1 was higher proficiency (HH) dyads; Group 2 consisted of mixed proficiency dyads (HL) and Group 3 was lower proficiency (LL) dyads. All the pairs rehearsed and then consecutively performed a problem-solving task and a debate task in two separate classroom lessons. The total data included 48 rehearsals and 48 corresponding performances collected during normal classroom hours. The results show that, overall, LL dyads encountered more language problems (more LREs) in rehearsal than HH dyads and they were less likely to resolve them successfully. However, they were able to use a majority of the correct resolutions in the performance as well as their higher proficiency counterparts. The lower proficiency learners were also found to employ memorizing and local rehearsing strategies to retain ideas and language items as they rehearsed for the upcoming performance. These findings have pedagogical implications for teaching and learning through tasks in EFL contexts and beyond.