98 Infiniti Q45

Car shakes at stop lights and tries to cut off, unless I put into neutral and keep my foot on gas peddle, What is causing this? Mechanic does not kjow what problem is.

First thing to do is find a different mechanic or pay dealer 100.00 to check the vehicle for the problem.

You need a new mechanic.
There are a number of things a competent mechanic could check out including the fault codes in the ECU, the IAC, the ignition system, and, considering the age, the compression. Definitely the compression. Throwing a vacuum gage on the intake manifold ain’t a bad idea either. That’ll detect vacuum leaks, sticky valves, etc. that affect manifold vacuum stability.

This one should not be rocket science to diagnose. Repairing the problem might be cost-prohibitive depending on what it turns out to be, but diagnosing it should be pretty straightforward.

By the way, how many miles does it have on it?
When was the last time it had a tuneup?
What tests/things did your mechanic do?
Did he read the fault codes? Do you know what they were?
Do you know if he attached any equipment to it to try to diagnose it? Do you know what?