89 TBI 4.3 Delayed acceleration

[jiimi62]

I have an 89 4.3 4x4 TBI s10. Recently I have noticed that when I accelerate hard she boggs down real low for a second. Almost like there is a delay then takes off just fine and accelerates great. This is only under hard WOT acceleration like trying to merge into traffic. It was running great before I noticed this. I do not get a check engine light nor does it show any codes.
I have recently put on a TBI spacer (didn't notice any prob until after I put on the spacer but it wasn't like as soon as I put it on that I noticed the problem. I have just noticed the problem since installing.) I also have a K&N open air element. It seems to run better cold, or after it has been sitting for a few. Is it just sucking up hot air from the engine and not getting enough air? Should I find a factory air box (it came with the K&N OAE)? Should I take off the TBI spacer (haven't noticed much gains so it wouldn't bother me)? I had a vacuum leak and drove it around running rich for a about a month before I found it. I changed out the O2 after but maybe it fouled my plugs? they are only 6 mo old. Any Ideas? thanks

[MT-2500]

A good tune up may help.
Go over secandary ign and fuel pressure and fuel filter.
Check exahust system for restriction.
Give us your engine vacuum reading at idle and at 2 K rpm.
How is that fuel pressure?
Good AC- Delco plugs?
Base ign timing good?