Speedometer quits or goes wonky off and on. Says I'm going 140 or 160 or doesn't work at all. Also ABS light is on and emergancy brake light is on.

All of those things might be related to one anther, then again they may not. ALL of those things are controlled by the Instrument Panel Cluster (IPC) module. The IPC turns the brake and ABS lights on or off depending on data that it receives from the Electronic Brake Control Module (EBCM) and the Powertrain Control Module (PCM).
The IPC displays the vehicle speed as determined by the IPC. The IPC calculates the mileage based on the class 2 vehicle speed message from the PCM. The IPC calculates vehicle speed based on the class 2 wheel speed message from the EBCM if a loss of communication or malfunction occurs with the PCM. The speedometer defaults to 0 km/h (0 mph) if the IPC detects a loss of class 2 communications with both the PCM and the EBCM.

To make all of this as simple as possible, you need to scan ALL of your modules for fault codes to help in eliminating possible causes for the problems you are experiencing.

Please note that MOST generic OBD scan tools CANNOT access all of the data information that will be required to properly diagnose this problem.

Also note that when really erratic or as you say, "wonky" things are happening, the VERY FIRST thing you should check is your BATTERY and CHARGING SYSTEM. I have seen many things like this happen because a battery has a shorted cell or an alternator has bad diodes, or both!

I had a Cadillac just last week that another shop had put a rack and pinion unit into it at a cost to the customer of $800 because his variable-assist steering was doing goofy things and was setting fault codes. The customer brought it to me when the problem continued and the other shop refered him to me. The techs at the other shop said they "ignored" the "Low Voltage At Module" codes because they and the customer allready knew that the battery was bad??? To which, I said WHAT!!!???
I fixed the steering by replacing the battery.

( The computer was suffering from dimentia caused by malnutrition )