2003- aftermarket radio shuts off after starting

[mcmccoy28451]

Hello all. First post here and new member.
I have a 2003 6 cyl with an aftermarket radio installed. About a month ago, the radio would shut off after starting the car. i noticed that it happens at exactly the same time as when the dash lights do their thing and turn off indicating all is ok. Now here is the weird thing. if I start the car and HOLD the key in start, the dash lights go off as normal, the radio goes off as before, but when i let go of the key to its normal run position, the dash lights all come back on for a moment (system check again?) and the radio will come back on.
I don't think its a power issue as the radio presets and clock retain their settings. Any ideas?? Thanks for any leads.