Weird Problem

[Roadrunner]

Today I took my GTA out on a 140mi trip (70 each way).
When I started out, it ran fine & strong for ~30 miles then it started running rough - losing power and bucking.
When I tried to give it gas I got a lot of detonation.

I found if I downshifted and kept it above 2000 rpm, it would run smoother but not quite normal power.
It would idle ok but maybe a little rough.
Between idle and 2000 rpm, it would fall on its face power-wise (guessing severe knock retard).
Going up slight hills requiring more throttle, it would do OK as long as I was above 2000 rpm.
On highway in 4th it would go 70~80 mph OK but for 55~65mph I had to downshift to keep it above 2000 rpm.
Below 2000 rpm, it could not hold 55 mph on flat highway.

When I started on the return trip, it was doing the same but not quite as severe.
By the time I got home, the car was back to normal and now I can't find anything obviously wrong (of course).

The last time I had it on the road was a fair weather weekend in January.
When I started out today, it had 2~3 gal of gas left that was put in there January and I put in another 8 before starting my trip.

Car has no mods.
Injectors have #523530 on side (not sure if this is multitech).

I also noticed that whenever I shut the engine off there is a gurgling noise that sounds like its coming from gas tank and lasts 5~10 sec (it may have been there before but I never noticed).

Has anyone seen this before?