I have had a 1991 Nissan 300zx SE for about a year now and just recently I began having a rumble in the engine when I am at about 2500 rpms (happens most often in fourth gear) where the car almost feels like it choking itself. The rpms get locked in place and don't change if i give it more gas and stops if I push in the clutch for a second. If I let it continue the rumble it lasts for about 5 seconds then goes away and I can feel a noticeable acceleration in the car. I lack enough knowledge to know what could be causing the problem and it happens rarely enough that I couldn't get anyone else to feel the same problem whether they drive or if they are with me. Anybody have a similar issue to this or an idea of what it may be?
Thank you so much for any help! I am new to this forum but it seems like a great place!

1st, welcome to NICE. Now as for your problem, it is very difficult to discern beyond just randomly naming things. I'm not sure what an SE is because the Z32 at least in the US was never offered in such a trim. If the car is automatic that may be the root of your issue (a malfunction with a solenoid or converter lockup). Follow the link in my signature to access the FSM which has a troubleshooting section that is quite robust and report back. Good luck.