Argh! Can't get my carbonation right for my kegged soda.

I recently made my first soda - 5 gallons of ginger ale. I kegged it and dropped it in my keezer @ 40 psi (~ 38F). I have hooked up 25' of 3/16" bev line and a picnic tap.

Yesterday, I tried a taste for the first time in a week or so. The oull was nice, and the ginger ale tasted perfectly carbonated. A few hours later, I tried another, and it spewed out mostly foam. All my carbonation was lost to foam. Since then, I've had the same difficulty.

I have a pack of these (link), and I can try them out in my dip tube, but shouldn't 25' be enough line for this situation?

25' should be plenty. Since you got a good pour once, I'd pull the dip tube and clean it.

[I keep my soda close to freezing, because I don't cool the glasses.]

Hmm - a dirty dip tube could be causing this? I cleaned the keg pretty well before putting the ginger ale in there, but I guess some sediment could have accumulated. I'll give it a shot. Thanks.