For starters I would go back to basics and look for poor and/or corroded connections. Disconnect the battery and then remove all the electronic multi-plugs one by one and give them a spray with some electrical contact cleaner and look for any obvious corrosion. I had a random starting problem on my previous Panda which turned out to be a poor connection from the throttle position potentiometer - cleaning the connector solved it permanently.
Check that all the earth connections are tight and free from corrosion.
Check battery voltage and charging voltage - strange things can happen when the voltage is too low or high.
You don't say if your garage has been able to pull any fault codes. If you are using an independent garage they may not be able to read all the codes and might just be pulling generic (non manufacturer specific) codes. In my experience many independent garages claim their code reader can do anything that the dealer level diagnostics can do but this isn't usually the case and it can be more cost effective to get the main dealer to investigate in the long run.
However, if you suspect the power steering I believe its possible to interrogate the steering ECU by adapting the pinout of a K-line VAGCOM OBD reader (available fairly cheaply from ebay) and using freely available software (e.g. from FiatECUscan.net) with a laptop.
There are lots of threads about failure of the electric power steering on Pandas and Puntos that you could look at to see if the symptoms are like yours.
Failing that I'd find an independent auto electrician who can perhaps install a datalogger onto the OBD port and see if anything significant shows up in the log just before the problem occurs.
Good luck
