First gear has loss of power

[JohnathanSins]

Recently got my 1993 300zx started after sitting for a year and a half!

The problem now is that when driving in gears, first gear is just so god damn weak. I know the difference in power and I can somewhat even notice it in the other gears as well but not to the point where it could effect speed capabilities (given, it is harder to reach speeds with this problem). It seems like RPMS are slower below 2000 rpms in first gear and afterwards, it’s completely fine. Rev response in neutral on the other hand is completely fine and in fact, is what I’m looking for in each gear.

I’m suspecting IACV valve needs to be cleaned but other than that, I have no idea where to start.

All advice appreciated!!!