Most likely your problem is poor contact somewhere in the battery/holder/
power switch series string. The radio gets enough current to receive OK,
but the heavy draw of transmitting causes the voltage to drop to the point
that the radio will no longer operate. Try a different set of batteries
(only one bad one will cause the problem), and make sure that all are
seated properly in the holder and that the contacts apply firm pressure
against the terminals. A bad power switch can cause the same symptoms.

I have used my Icom A2 with an aftermarket AA battery pack for years with
good results. Always carry a spare set of cells. They will keep for several
years.

Nicads are fine but you must exercise them. The right way to treat a Nicad
is to run it down then charge it up ASAP - at least once a week, and
preferably daily. Letting it sit for months will kill it, as will leaving
it on the charger all the time.