Do you get any warning lights when it does this? You might want to take it to a autozone or similar place just to see if they can get any codes. It might point you to the problem.
It seems like the aftermarket install messed the wiring or your car's system. You ought to bring it back to them.There are no warning lights and I have a code reader and I'm getting nothing. I tried to take it out this evening and it shut off every 10 or 30 seconds. Hardly made it back around the block. I then opened up the fuse slot and put my hand in to see if the OBD II port was in there (it's not, it's below that) and I hit some wires (spliced from the place that did my aftermarket autostart-through the dealer I bought it from) and the cars electronics turned themselves on all by themselves...I didn't press any buttons and the car was off!!! So I'm thinking there is a short, jump or cross in the auto start wiring in there. I then started the car with the auto start remote and it gave me warnings about the keyless entry, tire pressure and electrical service needed. Then the car wouldnt let me shut it off through the push button on the dash and instead i had to use the aftermarked autostart remote to turn it off. I removed the remote from my car (incase it was getting hit in my purse- despite the fact that I couldn't get it to shut off by pressing any of the buttons on the remote after I had started the car normally without it) and tossed around the wires in the fuse compartment and it's running normal again?