1998 astro awd rough running

I have what seems to be a common problem with rough running.

the vehicle starts and runs fine - engine light is on constant

after a short (1/2 mile or so) distance the engine starts to run rough and shortly after the engine light starts flashing.

It is possible to stop the light from going from constant to flashing if I drive with a very light throttle pressure.

The van had a tune up approx 2 weeks ago (rotor,dist cap, plugs, leads etc.) and went through the emissions test only last week with a near 100% pass rate on all items, so I am thinking I should be looking at other items than fuel delivery / tune up items o2 sensors etc.

Any help would be appreciated

A flashing SES (Service Engine Soon) light indicates a misfire. It's almost always caused by secondary ignition problems: distributor cap, rotor, plug wires, plugs or ignition coil. The 4.3L is very fussy when it comes to spark plugs, use AC Delco ONLY. Distributor cap and rotor should also be AC Delco. Make sure the plug wires are installed correctly: http://i561.photobucket.com/albums/ss58/cwhook/97newer43Lfiringorder.jpg The DTC's (Diagnostic Trouble Codes) need to be retrieved and posted here so we can help you. A quick check of fuel pressure and leakdown would also be in order.

HI
wasn't cheap but we seem to have resolved this issue,
I took the van into another mechanic and he read a misfire on pot 5 so he did a water pressure test and in his words when he'cranked the pressure right up, the cylinder was letting in water' hence the fouling and mis-fire.
he replaced the spider advising me that one of the sealing rings was letting by and so far, so good the mis-fire has stopped.

Coolant leaking into a combustion chamber can cause a misfire. If he pressurized the cooling system and coolant leaked into cylinder #5, the lower intake manifold gaskets should have been checked/replaced, (very common problem). The spider is actually the fuel injector assembly and has nothing to do with coolant.