My house was built in the 1930's and the wiring I'm sure is as old. The lights turned on tend to be on a dimmer they will be dim then go bright almost like an electrical serge sometimes flicker. Also I have a fan If set on med to high it will slow down then speed up.

What's causing this?

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The lights turned on tend to be on a dimmer they will be dim then go bright

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You said lights (meaning more than one).
You said it tends to be a dimmer (which means you are not operating the lights on a dimmer)<---if correct. Then based on what you said, I would get some one out there as soon as possible. Start by giving this information to your serving utility company. Tell them that your lights are getting dim, then very bright<--this could mean you have a loose neutral at the meter--which could cause a fire if the neutral was totally lost. There by sending 240 volts through 120v circuit loads!

While you wait, turn off all lights, and unplug all 120v general lighting loads. Then have some one check your service rated panel wire connections. You can do this, but I'm not going to recommend it.

Note: Your utility company should not charge you for this assumed emergency.
Note: If you are not comfortable with electrical, get someone out there who is, and have them check your service feed conductors immediately!
******even though this may not be the problem, we "must" assume it may be!

-Have you added any large loads to your home recently? Example: Hot tub; Electric furnace; dryer etc..
-Does it happen when a load is turned on, or opposite?

What type of fan is it and how are the speeds controlled?

Sounds like you have a bad neutral somewhere.