I'm assuming you're running a 1900 kv motor right? What pinion? If you're running smaller than 17, try that first. If it helps but still a little too aggressive try 18 and dial back your throttle EP to help with motor temps if necessary.
I would definitely start start there. I had the same problem when I first got my SCT410.3 and after trying a bunch of setup changes all I had to do was go up one tooth on my pinion. Much less invasive to try that first.

17t 1900. But this is happening just going around a basic corner at partial throttle. I'm driving it like a grandma because it's basically new. I have the EPA at 80%. I'm not beating the hell out of it. It just doesn't like the loose and bumpy terrain.

I'll go into a corner and without warning the rear end will rotate completely around the front end like the front was stuck on something. Then going down the straight it will just start flailing side to side. It's so unpredictable.

Maybe slide the motor out and make sure the drive train is free everywhere. Did you make sure the diff action worked properly in each diff during the build? If the rear end is coming all the way around like that I fell like something has to be wrong somewhere. It seems like that is more than a set up problem. But I'm no pro.

All is free. Going to go with a 7/7/5 diff setup to try to free it up a little. Read a few setup guides saying rough loose terrain should have thinner diff fluids.

If its super lose you might even want to try 7/5/3

Front Driveshaft
 

So, last week I had a major front impact when another buggy jumped the track and I hit it almost full speed on straight. After inspection, I found a bent A block, left inner hinge pin, and front drive shaft. I replaced these parts, and took the eb48.4 out again this week. After a seemingly minor bad landing from a jump (I've seen many worse landings previously without issue), I bent another front drive shaft. I can't see anything else wrong.

Any ideas what else could have broken to cause this? I can't visually tell if the chassis is bent, doesn't look like it anyway.

I might have to try this the track i run on is very loose and a little rough. Seems like i have no grip with 10/10/7

Did your front chassis brace break possibly?

do the arms fall freely? If you skipped the step of using an arm reamer or drill bit through the arms, the car will have difficulty just going straight. Hinge pins should fall through the arms with its own weight alone. Remove hubs and shocks from the arms and make sure they are free.

Also test the shocks after detaching it from the arms. Keep it on the shock tower and make sure it rotates around the tower and sways freely. Had a friend who tightened the nut on the shock standoff so tight that it crushed the plastic shock cap bushing causing it to bind up. The car couldn't even go straight, and on high grip the car couldn't even go 5 feet without flipping over.

Make sure each diff spins freely. Turn one outdrive and make sure the other outdrive on the same diff smoothly turns the other way. You shouldn't feel any resistance except from the diff fluild, not feel the gears grinding to the point that it has unequal resistance throughout rotation. Reason why I say is I had one spur gear fully tightened and would have a bad grinding feel. I knew the plastic gear might be a bit off, so I added an extra diff seal and after that the center diff was free. It happens because the spur gear and gear case is plastic. When I replaced the spur gear with another plastic gear there was no need for that extra diff seal to distance the gears a bit.

If those are good then setup is all that's left, but it sounds as if it's more than that.

I am assuming you are NOT running the center and rear chassis braces?

If you are its at least half your problem i think.

It's assembled properly and everything moves freely. I did not buy any additional chassis braces. But I installed all the parts in the bag...so are there parts I should not have installed?

Ok checked the manual...yes all braces installed. It didn't say optional so I put them all in. Arg. So remove the mid and rear?

stock setup- whats it? instruction manual or setup sheet at the end of the manual? they are a bit different - atleast braces and wing position, maybe something else. also, manual steering links length makes front toe postive, like 1-2* toe in front.

I didn't realize brace installation was optional until now. The manual says to install them. I didn't realize they were tuning options. All other adjustments are per the stock setup sheet.

I would start with removing the center & rear braces, also go to lighter diff oils 7-7-5 or 7-5-3

I still have the center and rear chassis braces installed on my buggy and have been tweaking setup myself. It seems most people recommend removing them.

I think I might do this to see for myself how much of a difference it makes.

My question is, has anyone bent a chassis as a result?