Truck Repair: 87 Chev S10 - 2wd - 4.3 - 5 speed, head gasket, oxygen sensor

Question
I'm about to give up - I love my little truck, but I'm at my witts end!  I read alot of your advice, and liked your answers.  Here's my problem. About a year ago, I replaced the head gasket, (yes, I had it machined as well).  Ever since, the truck lurches/surges when I drive at a steady speed, once the engine is hot. There is also some hesitation at takeoff with the fuel.  The warmer it gets, the worse it is.  The highway speeds are ok, it's the city speeds that are impossible. Just recently it has now started stalling when I engage the clutch at a stop light.  Here's what I've done to date:
-replaced fuel filter (twice)
-replaced all vaccuum hoses
-checked engine pressure - good
-used fuel injector cleaner - no help
-took throttle body apart & cleaned just to make sure nothing was sticking - no help
-replaced cap, rotor, plugs & wires (twice)
-changed oil filter (twice)
-replaced all other filters under the hood
-replaced oxygen sensor
-replaced MAP sensor
-replaced MAT sensor (twice)
-hooked to a portable diagnostic we'd get a code for the MAT sensor - but no change after it was replaced
-disconnected EGR valve - no difference - replaced it anyhow
-had complete exhaust system replaced - no difference
-possibly a few other things I can't remember anymore........
PLEASE VAN!!! CAN YOU HELP?
Any ideas?  I really do love this little truck, and aside from this issue, it really is a great truck that I'd hate to give up!

Thanks in advance
Wendy  

Answer
Hi Wendy,
Since you say it started at the head gasket change, I would expect something was done during the work.
I do very little internal engine work, but there may be a tensioner for the timing chain on that. It sounds like there is slack, and the chain is flapping, which, in effect, changes the timing.
It may even be difficult to time the engine, and keep it timed.
Another thought that comes to mind is the springs in the distributor advance system, but just changing a head gasket should have no effect on them.
The reason it would be affected by engine temperature might be that it has electronic spark timing, which may not be in effect during cold operation, but after warmup, it would try to correct the timing, but be fighting against the slack.
I don't know, but those are some ideas to check.
Good luck,
Van