So, all the sudden, both of the servos on my Losi Desert Buggy XL (stock Sprektrum servos) have gone to where they act completely erratic when plugged in, with no response to changes in radio signal. When they do this, the light on the receiver also begins acting erratically and if I plug other servos into the receiver, they also act erratic. But, if I unplug the two Spektrum servos and just leave other servos plugged in, they act completely normal. It doesn't matter what radio system or battery I'm using, things are completely fine until I plug one of those servos in. They had previously been doing this just off on and and I would switch the radio off and on again a couple times and they would go back to normal, but now I can not get either to act normal no matter what I do. Has anybody seen anything like this before?

Ck your Batteries, if using alkaline low batteries, NiMh it'll quit completely, Lipos ran them down slow low you cant recharge them.
Both servos at one time, I don't see it happening. Removing one servo and other one quiets down cuz servo takes power, Or remove both Servos and insert the throttle servo to steering, if ok, chances are the servo is fine, then remove from RX. insert the Steerin servo into steering TX if it chatters Servo is flakey.

I was reading about a week ago some of the Spectrum HV servos had issues, not sure if these are HV? The Losi gas buggies are the most reported from what I read. Doing weird stuff or just quitting. Have you tried another receiver? To eliminate that? I use the Spectrum servo in my 8 truggy. but not the HV version and in the last 4 trucks they have all worked flawlessly. I have the bec turned up to 6.5v and considering bumping it to 6.7volts later. It has no issues with yanking the truggy tires around. What's the model # of the servo?

Oh yeah, here's the weird part: the servos are spms901's, which I can find no evidence of even existing. They appear to just be a metal geared version of the s901 though. I've tried with spektrum radio and flysky, only plugging in one servo or the other to different channels, always the same result. I don't think they're hv and I'm running a 6v battery.

Yeah the ones I use come back to a different model # too. I was really surprised by their durability. It sounds like you have eliminated everything except the servo itself, I would call it and either replace them or see about some type of warranty.

Horizon is sending me two new servos even though there's no warranty or anything like that, which was super great of them, but the conversation I had was super short so I'm not really sure whether I should be expecting the metal gear spms901's or the plastic gear s901's they show on their website. Either way I guess I should be happy since they didn't have to do anything. This is the second time I've dealt with Horizon support and it's been fantastic like this both times.

Man that's sweet! Do they want the old ones back? If not, I would hang onto them for the gears.

No it was so weird. He was ending the call with the usual stuff and I asked, "do you not want these back so you can see what happened to them?" and he said, "no, you can use them for target practice, Christmas ornaments, whatever you want, we don't need them." Some companies make you destroy the old ones, others want them back, Horizon sends you $200 worth of product and tells you to use the old ones as Christmas ornaments. Top notch. I'll definitely swap the gears if the new ones are the plastic gear version.