My truck has been doing this lately and I can't figure out what the problem would be. Let me explain when, and what my truck does.
Going around town, the speed limits here are either 25 or 35. 25mph is when my truck shifts into 3rd, and 35 is when it shifts into 4th. Whenever I'm going through town and I'm going the speed limit, is right when my truck shifts up into the next gear. At 25, or 35, my truck shifts and I keep at that constant rate of speed where my truck is just kinda coasting along at very low rpms. if I keep at that constant low speed/rpm, eventually my truck will start to buck back and forth and i can hear clanking while its doing it. ANY, and I mean any change on how much pressure i have on the gas will make it stop. Accelerating and slowing down it will NEVER do this. or if cruise control is on the freeway.

Recently I've changed my rear u-joint which was causing a lot of problems and weird noises, but this wasn't one of the things it fixed.

Before anyone gives me some suggestions, let me explain all the new parts i've changed in my truck recently.
EGR Valve, Distributor, Dist. Cap, Spark Plugs, Spark plug Wires, Distributor rotor, MAP Sensor, Ignition Coil, rear u-joint, Air filter, fuel filter, and fuel pump (pump was a while back). I've checked all hoses, and fuses in my truck, Tranny fluid is newly changed (with tranny filter) and still got plenty.
This isn't new, my truck has been doing this for about a month. Any help or suggestions would be greatly appreciated thanks in advance

 

Anyone ever had this problem before? or have any ideas on what is causing my truck to do this? i've tried searching through hundreds of posts trying to find an answer or similar problem and I can't find one.

 

Poor injector spray? Worn TPS sensor?

 

my moms 95 LT has this with the "W" motor. at like 55mph it has it. To me it sounds like "chuggle" like the clutch discs are slipping in the 4l60

 

thanks swartlkk, i'll try checking out the TPS. I've never heard of this part until now and I just finished reading up on it quite a bit and it sounds like this could potentially be my problem.

1994BlazerS10 - this doesn't ever happen to me if i'm going above 35. when it does happen though my truck bucks and i can hear some nasty clanking from underneath my truck. So anytime I even feel that slight first jerk i just step on the gas and it will stop.

this problem really really blows because it prevents me from just being able to cruise along at 25, or 35 miles an hour. I feel like i need to speed everywhere now just so it doesn't do this.

Also, my truck doesn't seem to do this at all when its cold. Only seems to do it after driving around for 10-15 minutes.

 

been a little over a month now, have you figured it out yet?

your '91 sounds like it was doing the same crap my '98 was for the last 6 to 7 months. turned out to be the timing was way off (like 15* off).

don't know who screwed with my truck in the past (looks like nothing has ever been touched back there), but just under 200K & almost 12 years, how could it have run so long like that? just one of the things when you buy a used vehicle, but i'm never gonna buy a 1 (female) owner vehicle again, that's for sure.

pull the wire under your heater box, and check your timing, if you haven't done that already.

 

Sadly, my truck still does this sometimes. I've replaced a lot of things pretty recently. Checked my timing not to long ago and it was off by a bit, corrected it, and it still does it unfortunately. Never have any codes or anything.

 

i just bought a 93 blazer and it does almost the same thing, only mine does it when i put it in overdrive and hold the gas constant, its the transmission having problems shifting, im not sure what causes it but the jerking is the transmission shifting in and out of overdrive