Rough startup when cold, P0300-P0303 codes

During startup, my engine is misfiring, and it's getting worse, up to a minute last night and it through P0300, 301, 302, and 303, the misfire code. When I press the brake pedal, the roughness settles down a little, and when I life my foot off the brake, it gets rougher. If this were an older car, I'd look for a vacuum issue, but iirc, the brake booster is electric and not vacuum powered. I know the ignition system is one of the first candidates to check, but based on the behavior with the brake booster, does anyone have any recommendations on what systems to check first?

Edit to add, since know head gasket is something I need to look into, possibly with a borescope:
1. Does a failing head gasket usually affect 3 cylinders? Everything I've read up to now suggests in starts in cylinder 1 and spreads to 2.
2. No observed coolant loss, been watching for over two months. Would it be like this during early hg failure? Would a borescope pick it up?
3. I've read it's usually a problem that shows up as roughness when you've let the car sit overnight. This happens after the car has cooled for an hour or so, which doesn't seem like enough time for the coolant to fill the cylinder and affect ignition and compression.

I'd like to check everything I can before taking it to someone with a scope. Thanks

 

Considering it. How do I find someone to take it to with a scope? I also have a few things I'm wondering:
1. Does a failing head gasket usually affect 3 cylinders? Everything I've read up to now suggests in starts in cylinder 1 and spreads to 2.
2. No observed coolant loss, been watching for over two months. Would it be like this during early hg failure? Would a borescope pick it up?
3. I've read it's usually a problem that shows up as roughness when you've let the car sit overnight. This happens after the car has cooled for an hour or so, which doesn't seem like enough time for the coolant to fill the cylinder and affect ignition and compression.

Asking these questions because I'd like to check everything I can and then take it to someone with a borescope. If it's the hg, I'll try to replace it myself, because that's a lot to spend. Thank you.

 

170+ and 7 years.

 

Hmm, saw a borescope on clearance at Advance auto so I went ahead and bought it. Going to pick it up and then look in my cylinders.

 

Thank you for that answer. I'm going to check the gasket for sure, and I guess look into what else could be happening if it comes back ok. It's a shame toyota didn't do a replacement program for the head gasket.

 

Update: Got ahold of a simple scope, but just pulling the plugs, I saw no 2 cylinder's had a white tip. Additionally, looked like there was steam or vapor in cylinder 1. None of the pistons looked clean, but I guess it would with time. I know what I'm doing this weekend, leaving car as is until the parts get in and I replace the head gasket. Wish me luck. First time, but I've been working on cars since I was a teen. Thanks everyone

 

That makes sense. Will do that tomorrow before I take anything apart. I found this bit of advice and wonder if you think this is a good method, it makes sense since the pistons are paired 1-4 2-3 at the same depth if I understand correctly.

 

i'm gonna have hope that mine isn't too far gone. the rattling is recent, and I haven't lost much coolant, so much that I haven't noticed any loss over several months. That makes sense about the MGs, although I thought only MG1 was used to start the engine.

 

Oh, I will add that my car is in the garage, not moving anywhere until the parts come in and I finish the job.