There are several things it could be, but the most obvious and common is a bad battery. Most people incorrectly diagnose this as the alternator, however, it is in fact a bad battery. The battery is not holding a charge and is constantly placing a full drain on the alternator produing your flickering/pulsing. Check the voltage on the battery with the engine off immediately after running the engine for at least 15min. Wait a few hours (overnight even better) and check the voltage again. Anything more than a tenth of a volt drop or even really hundredths or so and you have a failing battery. For ex. right after running if the battery reads 12.5V and then after say 8-12+ hours if the battery reads 11.9V you definitely have a failing battery. If it does not seem to be a bad battery, repost and we'll go from there on less likely causes.