Low engine power?

Car spits and sputters. The check engine light on flashing. Smells gas bad like rotten eggs. White smoke out of tail pipe. Not running hot only half way temperature. Check engine light went off for a few days an ran perfect then light came back on and it has no power. Press gas to floor and it bogs down. I am stumped.

Ok so I've been battling this problem for a while to no avail:
My 07 impala 3.5 flex is making valve train noise after replacing intake manifold gasket, and replacing plugs and wires I've been told it may have something to do with pushrods and or hydraulic lifters here's my testing so far

Compression
1-198
2-202
3-195
4-203
5-201
6-200

Fuel pressure-
steady 58 psi running holds 50psi after key off
Fpr- operates fine

Vacuum-
Cranking 5"hg
Idle- 19 average but constantly fluctuates/flutter 3-4"hg
Snap throttle-0 and 25"hg high back to 19"hg slowly with a erratic fluctuation
1000rpm-20"hg pretty steady
2500rpm-22"hg

Good spark
Ac delco iridiums
Plugs gapped at 0.040"
Clean filter

Car has a weird loping feel at idle (very slight)
Can here constant ticking in car at idle
Idle is normally at 750-800rpm but intermittently it will drop about 100rpm or so when brakes are applied, and the engine begins to lope
I experience very inconsistent acceleration, loosing speed up hills
Doesn't want to hold rpms cruising in 2 or 3

The tranny isn't in the greatest condition but it has driven pretty much since the day I got it, and my problems got way worse after the manifold gasket replacement which is why I'm thinking valve train issues

Any insight would be appreciated

Well see that's where I'm stumped I personaly did the intake gasket, all pushrods in correct order, no wear on the gasket the first time I did it, second time I checked everything over againand I couldn't notice anything wrong, only thing I noticed was that the number 2 Pushrod looked a little stressed maybe bent very very slightly, but I don't see how that would cause such a lack in power, and its not missing.

I also don't think it has to do with the pushrods necessarily bc I have driven about 4000 miles on it like it is

Any other ideas?

Also the rpms will jump when I LET OFF the gas cruising at like 45.
I just want to eliminate some possibilitys
A collapsed lifter would be noted in a loss off compression correct?
And if the Pushrod was indeed bent wouldn't it missfire?
Is there any way a rocker arm pedestal could be installed the wrong way?

I mean jump as in 200-400 rpm once and then it decelerates as it should

No codes, fuel pressure at 58 steady, and fuel pressure responds as it should on acceleration, throttle plate cleaned alreAdy, and there is no iacv on this car, its a drive by wire so the throttle body is electric, and tps/iac are basically one unit

First off I'm sorry for asking a bunch of questions on this car I just want it fixed so I can get rid of it.

Before the intake it was running funny, hesitating on take off and hesitating on accel, I tore it down after it started leaking coolant from the crossover.

After I replaced the intake and plugs/wires the car started funny and was noisy on startup, the valve train started making more noise then before and my power loss/ hesitation became worse

Basically my question is.
Could this be an out of place Pushrod?
Could I even drive 5k if I did misplaced a Pushrod?
Lifter issue?
Or a clogged cat?

I ask this bc all I have tested so far came out good except manifold vacuum
Compression 200 -/+ 5 all 6
Fuel pressure 58 to about 60 on accel
Spark strong
Fuel Injectors all working
Air filter new
Maf/map good
Coolant/iat good
Throttle clean
Plugs(ac delco.040)

Only thing standing out is vacuum fluctuates 2-4 at idle but OS pretty steady at 19

Any ones knowledge on this homogulation of the 3.4 please help me out, I'm stuck between a rock and a hard place I would tare into it again but I want to be 110% sure of what I need to fix

Thanks again

I also searched everywhere for a vacuum leak to no avail, the ST AND LT fuel trims don't show any evidence of any cylinder(s) running lean due to a vacuum leak

Couldnt that fluctuation be from the lifters sticking? And I take it if a valve got burned it was most likely due to the lifters sticking?

And can you explain a good prep for installing the lifters?