Nothing to do with the chip. The decel and idle should actually be significantly improved over the stock chip. Factors that will cause stalling are:
1) too low of a base idle adjustment
2) base idle mixture either too rich or too lean. Idle CO should be between 0.6-0.8% before the cat
3) intake air leaks causing mixture problems and thus mis-metering by the air flow meter
4) gummed up idle control valve
5) intermittent or non-activating idle position microswitch
6) oil and dirt accumulation around the inside of the throttle body where the throttle butterfly closes
7) ultralight flywheels or aftermarket cams such as 964 or 20/21s, in which case a different chip is needed
If the stall is not severe, increasing base idle speed by turning the screw counterclockwise 2-3 turns out may fix the problem. Otherwise, clean items 6 and 4, check item 5, check item #3 by disconnecting the O2 sensor and connector to the ICV, and spray engine starting fluid at the intake manifold gaskets, injector O-rings, manifold boots, vacuum lines, and anywhere you could suspect a intake air leak. If the RPMs rise rapidly, you have located an air leak that needs to be fixed.
Then check and address item #2, either with a CO meter at your local mechanic, or using the reading off the car's O2 sensor with a voltmeter. See:
Idle Hunting -- The problem continues!!
Finally adjust item #1, by using the factory method of jumpering B and C of the test socket, and setting idle to 880 rpm. See the bottom of the following page:
http://www.911chips.com/dmeconvr.htm
Properly set, on decel, the rpms should not subdip below 880 rpm and if it does, raise the base speed. Conversely, when warm, the engine should not idle above 900-925 rpm, and if so, turn down the base speed. Properly set, with my chip, you will see on decel, the rpm dropping like normal to 1050 rpm, after which, the rpms will float gently down to about 880 rpm in about a second, which gives even old worn ICVs time to catch and stabilize the idle.