Rentals - whether it's single family or multi - all have one investment strategy in common... someone else is paying your mortgage. In the long run - you should end up with very positive cash flow... even if, in the meantime, you had to replace the roof... paint... and carpet.. and fixtures. At that point - you had a bad tenant - it took you 6 months to evict - and he damaged the place... you're not very happy with that investment. BUT -- If you know that in the long run someone is going to pay off your mortgage and you're going to end up with positive cash flow... You'll fix the dump up - re-rent it - and keep on going. And that's my real point - and Mikes point... You must have a basic belief that you're on the right track and be willing to suffer sometimes for the bigger picture (the paid off mortgage or that trust in the long term chart!).
Stocks are very similar...
If you have one - and it goes south -- you're not very happy... but if you have 10 and some are doing fine - while others are sitting, but are okay - and one of them goes sideways... it's no biggie. There is no loss of capital unless you sell below your cost... so for the time being it's really only a mental challenge to trust your judgement as to why you bought in the first place.
I remind people time and again - when in doubt - go back and research your reasoning. Would you buy the stock again? Do you still think "X" is a leading brand - is it a good company - is the chart good - is the dividend good... Look at the details of the chart and see how many times it's gone UP and DOWN... Right now - you might be in one of those little down squiggles... If everything else is okay (no accounting issues - no legal issues etc) then instead of thinking about selling - maybe you should think about ADDING to your position and take advantage of the dummies that are selling!