Not nearly enough information. What instrument libraries and instrument are you using? What plugins are you using? What is your hardware setup? How are you exporting audio?

Thanks for offering to help! Hardware: macOS Catalina 10.15.7, 3.8 GHz Quad-Core Intel Core i5, 8 GB 2400 MHz DDR4 memory, Radeon Pro 580 8 GB Graphics. I'm using a Scarlett 18i8 audio interface, but the problem occurs when using Built-in output too. The problem occurs with Garritan bundle, GPO-5, and Softsynth, and midi output (played through audio units and midi). The audio unit output uses the bundled Aria Player - I presume midi output uses the internal Apple midi instrument(?). The dropout happens very rarely on playback (after upgrading to Catalina) but frequently on export to audio. I'm using File --> Export --> Audio File to AIFF (haven't tried WAV). The occurrences appear to be random: never in the same spot and not related to the density or volume of the audio. For example, I've had dropout on a single piano line and on dense passages. I've tried shutting down all other apps to no avail.