it might be that your pc is drawing to much power and the battery is not charged enough. but what i suggest, if you can is get rid of the isolator. they drain power and charge seperatly your bateries, and you will always have that problem. instead get a relay. a heavy duty realy from an auto electric store. hook that up and it will charge both bateries at the same time.
all you do is hook a line from the starter battery to the relay on one side, then hook the other to the second battery. hook a ground to the relay and then a remote line from your stereo or such, so the relay kicks on when your car turns on thus letting a charge go to the second battery aswell keeping them both nice and full. thats how i set mine system up and im useing 3 optima yellows and only need the one relay to charge them all, havet had a problem for over a year now.
you can buy one online. its a 200A relay or more if you can, oznium has this one but its kinda spendy. i got mine for 25 buck at an auto electric place and i sugest u get one like that too.
This 200 amp relay can be used as a battery isolator. When you use more than 1 battery in your car, you want to isolate them when the engine is not running. This prevents excess wear on the batteries because they will not be able to drain themselves.
" from there site
and i put pictures to show u mines so u can see that it works for more than 2 batteries and it works great and its cheap
and i ran only a 4 guage from the front to the back. so its easy cheap and efficent.
try that. it shuld solve your problem.