Hey, Welcome to the fun state 
Bursting full auto can be a couple of things, depending on a few factors. The Dytac SLR is a blaster I’ve not worked on yet so I don’t have any direct experience or information to them, but I am led to believe it is a CYMA metal gearbox, which I have loads of experience with. It is odd for semi to be unreliable in a cyma metal box, or any gearbox in 7.4v, assuming you are using the covered by warranty battery (don’t mention it to them if you didnt!)
My first advise is to take it back, and make it their problem, exchange it, or have it fixed.
Semi auto on a trigger trolley to be reliable cycling requires you to pull the trigger, finish the cycle and then reset and pull again. Some blasters (not usually cyma so much but it can happen on any v2 trolley system) will fire auto if the trigger is lightly pulled, or “creeped”. If you are spamming the trigger with semi on a stock blaster, you are in for a bad time, learn the cycle time, and to pull in full and deliberately, and reset before repeating. It is just a limitation of the trigger trolley setup. Seen way too many people ruin blasters this way, so be mindful of this.
If they are using a cyma mag, it is usually the wires that ruin their mags reliability and feeding, they are awful and the actual wire is thin and gets breaks usually where the feed door is. Replace the wires in the mag with something decent and they usually spring to life. I personally, have loads of network cable, and strip it out and use a pair out of that. Once the wires are replaced they will keep up with at least 30rps no problem.
Generally, checking the cut off lever moves freely, replacing the cut off lever spring can help, but only if that is the problem.
Trigger trolley in cyma is usually good for semi reliability, but like the mags electrically is their only downside, the grease from the tappet plate and cylinder can glaze over the contacts creating resistance and more leads to the trigger going a bit dead. A good clean with alcohol and a light file usually sorts that out.
Beyond that, a proper m90 spring, will get the same sort of power as stock but is a bit longer and should give enough resistance to make semi more reliable. Given they are rated for around 300fps, again probably a cyma standard metal box spring. They are kind of like an m100/m110 that is cut a bit short. If you fire on semi and hear a bit of a “boinging” sound after it cycles, its a good sign of a spring that is too short. But again, not usually a problem in cymas, but something to note.
If you intend to do a lot of semi only playstyle, I would look into getting 13:1 gears with a slow to medium speed high torque motor. something around 20-25k. It won’t give a large boost in auto rps, but semi will be snappy af and you will be able to reliably spam it. You won’t need a mosfet, unless you want that hair trigger. Also worth having a larger spring and short stroking to suit the barrel and desired fps will help with semi spamability. I have done dozens of cyma metal boxes with m110-m120, fps from 300-380, and rps from low 20s to 36, and never had issues with them, the customers only came back to buy gels 
I would recommend changing to deans plug and removing the fuse once you start upgrading it, as tamiya plug and the fuse will be a point of resistance.
