Terrible Gas Mileage and Throttle Hesitation

[WhatIsThisIDont]

So in this cold, I'm getting around 13mpg on 93 with 50/50 highway and city driving, most trips being over 10 miles. One tank that was about all highway was around 19mpg.

When I'm at a dead stop and floor the accelerator, the power light comes on, but the car just sits there for a full second, and then it takes off. Once it is moving, the car doesn't seem to have any power issues.

So far, I've cleaned the MAF and the Throttle body. I've also swapped the fuel filter.

Other things:

the PO replaced the air box with a go-fast hot air intake cone.

There is no CEL.

My temperature gauge now varies slightly, but usually stays around the 25% mark, with a max variance now of about 10%.

The car has 112k miles. The previous work is mostly unknown. I was told the transmission was replaced with another SVX stock tranny. Motor seems to be entirely stock, judging by the oil oozing from every seal and gasket.

(EDIT: I forgot about my varying idle. It doesn't happen often, but occasionally the idle will flick between 200 rpm and 1200 rpm. Fiddling with the throttle for a second sorts it out)

Any idea of how I can go about diagnosing this and maybe raise my efficiency?