Slipping, vacuum leak, sensor???

[W00I30]

2000 Infiniti i30 140k miles. Long story short, I get a slip when the transmission shifts from 1st to 2nd gear. I always thought I had a transmission slipping problem because of the bad solenoid pack that is a common problem in our cars or a drop sensor. Then the other day I noticed something different.

I had the car in park, pressed the gas like I would normally drive. The rpms reved up slowly and then about 1-2k RPMS started fluctuating and slipping in park. The rpms kept going up and down up and down until I gave it more gas or let off the gas. This is exactly what happens when I am driving and I always thought it was the transmission slipping. It only happens at low throttle. If I give it more gas it does not do it.

Does this sound like a transmission problem or and engine problem? Could it be a bad sensor like a TPS or vacuum leak? Its weird really because some days the slipping is a lot less so I'm thinking it's not the transmission anymore but an engine/electrical problem?

[Ilya]

Sounds like a vacuum leak, MAF, etc. not a transmission issue.

[NutriaforBreakfast]

yes it's really easy to think the transmission is shot when you really have a vacuum leak
yes tps maybe or IACV