When solo mining, can I stop and start ethminer once in a while (a few mins gap) or will this reduce the likelihood of finding a block significantly (i.e. starting all over again)?


You can stop and start without hurting your chances very much. Mining doesn't take a fixed amount of time, there's no progress to be lost. Every 15 seconds you have a relatively fixed probability of finding a block, regardless of when you started mining.

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