Check engine light comes on intermitantly, mostly at low speed. performance does not seem to be affected. This is a high mileage vehicle (250,000 miles)

I'd start with a thorough check of all your engine area harness connections making sure the multi plugs are properly home. Then any ground cables for a good clean connection. Has it had any work recently that could lead to a trapped part of the harness? I can't say whether the poor running is part of the electrical gremlins but as most engines have lots of electrical devices, who knows.

Check engine lights have been used for over 30 years and yet most people do not know what they are used for.
The check engine light tells the driver a problem exists with the performance or emissions of the engine. Not the actual engine itself.
Anytime something happens that could affect emissions or performance, the light comes on and a trouble code is set in the computer memory. The driver then has to have the computer scanned for those codes to know what component to have checked.

The check engine light is telling the driver the computer has a problem with something that can affect performance or emissions.
The problem is serious when the light is flashing.
You would need to have the computer scanned for trouble codes to proceed.

Codes that light check engine lamp are emissions/engine performance related only. Your DRL and SDM both talk to the ECM for inputs (they need to know if its running), but they dont affect the engine or tell the engine module to change how it runs.
Your ABS can affect how the engine runs as it sends wheel speed information that is used to adjust engine output, transmission shifting, and fuel/timing in some circumstances.
Check the ABS module for signs of water or oil affecting it, or water in the connector plug. Also check the harness where it leaves the module to make sure it isnt rubbing thru anywhere you can see.

GM ABS modules are not uncommon failure items, however mostly they set module fault codes or motor amperage/function codes. Id start with further diagnostics of the ABS.

NOTE: using generic OBD2 scan mode will show you only codes that are check engine lamp related if you want to be sure. Enhanced scan mode shows you any and all, and some of them simply have nothing to do with the engine lamp being on.

The fan clutch may be different from the original, they use oil inside to vary the speed most of the time. Some are more affected by air temp than others.
As far as performance goes, is the check engine light on ?

Ususally when they come on and go out peroidically it is fuel related. Low octane fuel will give a bad oxygen sensor reading triggering the light for short periods. (Modern engines are recommended to use mid to high octane -grade fuel) Also a bad gas cap seal or not tightening it properly after fueling can cause that. (Manufacterers recommend replacing the gas cap every 2-3 years) Whatever it is , it is affecting your engine performance to some degree or the light would'nt come on. See if you can get a code out of the computer, that will give you a better indication of a potential problem.

The harmoic balancer is located on the end of the crankshaft. This shaft is the largest shft in the engine and should be the one where all belts eventually run thru/over it. Check it out, let me know.
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cleaning the sensor may not have fixed problem --also check connections with abs and speed sensors but recheck code and if still maf possible wiring ir sensor failure

look for a diagram under the hood follow the route for each vacum . you have a intake leak take your time and go over each side of the engine even plastic air line could be cracked

Take a gauge and check you oil pressure. If its not low then change the pressure sensor. If the pressure varies and the pressure is not where it is supposed to be, then drop the oil pan and look in the bottom of the pan. If you see slivers of metal then the bearings are going out. Eather the cam bearing, rod bearing, or main bearings.