Be amazed at how many 24 year old females think that those $20 receptacles combined with the $150 chrome coil pull down kitchen faucet mean they MUST have this specific place..

Also, it could be dirty coils not allowing the water to drain down the coils properly to the drain pan/pipe. 

While they are they, have them vacuum the coils (if it is an older fridge with the coils on the back) and under the fridge ... and then remind the tenant they should be doing this 2-4 times per year.Alternatively, have them compile their service wish list and make a single visit to attend to it all.

It could be almost anything ranging from a weak breaker, a short, high amp pull from the compressor due to dirty condenser coils, high amp pull from the compressor because it is going bad, absence of a 3 or 5 minute relay so the unit cycles to early, etc.

I recommend replacing it with a standard coil-top if you can afford it.

In Pittsburgh I pay $2,000 for new AC/coil. 

. --------------------------------------------------------------------------2nd question:3 of the 4 units have very dirty coils and ducts.

The central air unit/coil was newly installed a few months before I bought the place so I'm not expecting/planning any capital expenditures on the unit. 

That coil top stove also makes me feel dirty.Triple the photos also add inside the garage and the bathroom/ basement if there is one.

If your paying a lot for labor it might not be worth it, but if you have basic abilities, a mother saw and trim nailer, you can knock it out in a day and make a statement.