it's a 99 s10 blazer 4.3 vortec 4l60e 4x4 4 door whenat crusing speeds of 40 to 50 mph if i leave the rpms drop down it will start to surge/hesitate/jerk it seems to do it when you leave the rpms drop to around 1500rpms on a slight load light throttle not noticed if rpms are above 2000rpms or if givin more gas or put it in third but that puts it about the 2000rpms.It feels like torque converter lockup shudder to me

 

you might have more than one problem...do you have an engine light on? have you ever inspected or replaced you lower intake manifold gaskets? hows your distributor? when was your last basic tune up done?

 

the tune up was done about 6000 miles ago and it has had the motor and trans replaced about 85k ago i bought it about 6k ago and finished the tune up they started but didnt really do me justice they cracked a plug and had the air filter hanging half out of the air box how you would do that is beyond me it is setting no codes but when i bought it it had a few problems that were mechanic's errors when they did work to it they messed up as mutch as they fixed but i thought i had them fixed for the last 6k it leeks no antifreeze and has 22" of vacuum so i would say the intake gaskets should be fine it has 180psi compression and distributor is suposd to only be 7k to 8k on it and all vacuum lines have been replace recently

Im goin to check into a few things i found on the forums one guy said he fixed his with a tcc solinod and that was my first guess and when i call trans shop to ask what they thought they said it didn't sound to be trans related even though I told them it seemed like the converter was shuddering and another guy said he fixed his also with a MAF sensor so im going to do tests and will keep posted

 

I tried a ignition control module today my buddy had that didn't help but it seems to be leading to tcc solinod at 40 to 50 mph or so when the rpm's drop down below 1500rpms or so at light throttle is when it starts if i tap the brakes it seems to stop but rpms go up to about 2000rpm also witch is above the problem area but right back to what my hunch was in the first place.

about 10 miles before this started i filled my tank so i took it and put lucas fuel treatment and stp gas treatment in when i toped it off with 93octane but it was not very low and could only fit 4 gallons max but that didn't seem to help

 

my miss got worse it was doing it on the interstate at 60-70mph i cleaned the throttle body,map sensor and maf sensor changed the airfilter that was new with about 6k on it but due to it being installed wrong when i got the blazer it messed up the seal on it and i changed then autolite professional plug wires that only had about 6k on them also if you looked under the hood at night with the under hood light unhooked you could see the spark inside the wires in places they burnt the wires out inside the insulation you could hold the wires were you seen the fire and it didn't shock you but running alot better seems to be fixed.

 

Glad you got it fixed! These types of problems typically are ignition related as you have found out.