Stumbles and backfires during heavy load

Lately my Astro -87 has started to stumble and miss hard and even backfire when I accelerate heavy with wide open throttle. It mostly appears after speeding up and the transmission changes into third or fourth gear while the throttle is still wide open. As soon as I let go of the throttle the problem disappears and the engine runs smoothly again. The problem seems to come after driving uphill or with higher rpm for awhile.

I’ve changed plugs, wires, cap and rotor and I thought the problem was gone. But when I accelerate hard for a longer time (uphill mostly) the problem still is there. I’ve also checked the EGR and it is functional and doesn’t stick.

It idles smoothly and performs well during part throttle conditions.

Any suggestions?

(Sorry for my bad english)

Regards,
Tony

Have you checked your fuel pressure? Sounds like it might be running lean at WOT.

Sounds like timing to me.
might be the knock sensor.