Pool pump starts randomly (as if there is a short) even when it is not set to start and stop. Otherwise works fine, not sure if I need to replace unit or dissassemble and clean or ????? T21004R

Sounds like 240Volt WG1573 clock motor has gone bad and needs replacement. But more checking is needed.

You posted under T2100-4R which is 60 amp Intermatic control panel with a single T104M 240Volt timer mechanism.
All Intermatic timer mechanisms can fit into same control center, so you can possibly have different timer mechanism inserted into the T2100-4R control center.
The control center can have up to 4 circuit breakers.
Instruction manual shows that timer can also have fireman's switch that is wired to pool heater and causes pool heater to turn off 20 minutes before pool pump turns off.
http://www.poolandspacontrols.com/products/mechanical%20controls/60%20amp%20pool%20and%20spa%20control%20panels.aspx

(1) First thing: If you have T104 mechanism, do a troubleshoot run-down to see if trippers, clock motor, or wiring are suspect:
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timer.html

(2) Next would be to check the circuit breakers for loose wires, burned connections, or the smell of burning or crackling sound that says breaker is loose on the busbar.

(3) Check for loose wiring on T104 and test circuit:
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.html
(a) Push the manual override lever to OFF position and test across terminals 1 and 2 and then across terminals 3 and 4 to make sure timer is turning off power to pump. Both tests should show 0 volts.
(b) Push manual override lever to to ON position and repeat test above. Both tests should show 240 volts.
If the test fails, then buy new T104M timer mechanism.

(4) If the T104 timer is turning off both lines in the 240Volt circuit, then pump cannot turn on since neither wire is Hot.
However if your T104 timer is actually a T101 timer or if timer has different wiring, where timer is turning off only 1 leg and not both legs of 240Volt circuit, then pump has one hot wire available and a short to ground could cause pump to turn on unexpectedly.