Backfire when quick throttle
 

Hi All ..
I am new to the group and did a search and really couldn't find any posts about a specific problem I had yesterday when driving my caddy.
It is a 1999 Eldorado with about 80K on it and I don't drive it much but recently I have put about 500 miles on it or so with no issues.
Yesterday when going up a small hill it seemed like it had less power than normal and almost seemed to be missing..
Anyway when I got it home in the driveway I was giving it throttle and noticed when I gave it a quick full throttle "pump" it was not smooth at all and was even popping through the intake.
Would this just be a tune up needed or map sensor issue or could there maybe more of a major issue of the head gasket or something?
I don't smell any gas or antifreeze out of the exhaust and it doesn't use any fluids except a little oil once in a while which I believe is normal for these engines.
I haven't done any tune up on it since I've owned it and the car does sit a lot for months at a time inside a garage but this came on all of a sudden and I have no check engine light at the moment.
I did have a "knock sensor" code a few weeks ago but it reset and I haven't had any problems since...
Any help would be appreciated
Thanks