I'm flying a heli and a fixed wing plane from the same Tx with duplicate receivers. The plane flys OK, no problem, but the heli occasionally "jumps" just like a person would when getting a static jolt. I'm looking for suggestions on what the problem could be.

The setup is that I am using a 5-cell NiCd battery (unregulated).
The Rx antenna is routed well away from everything else.
All metal parts (except bearings) are bolted tight.
The receiver is well padded as is the battery pack.

I've tried tapping on various components (receiver, servo, battery, switch, gyro and mechanical assemblies) with no effect. The problem only seems to occur while flying, and the problem can happen at any distance from 10 to 200 feet. Since a heli is basically a flying Cuisinart, it makes me very nervous when this lapse of control occurs.

Am I overlooking something?

A heli can glitch from many sources , and can be very difficult to find. Anything from a bearing spinning in it's seat (try a tiny drop of CA ) to the belt in the tail ,to a carbon fiber tail boom (causes a lot of trouble) though i'm not familiar with the exact model heli, or TX your problem is a common one. Some times a heli will fly just fine out of the box and the first light crash it glitches, and some time they have problems right out of the box. Sounds like a harmonic glitch caused by a moving part . If you cant isolate it you might want to try a PCM receiver .

Have you tried replacing the RX and or crystal with other known working components?

Helicopters are well known for generating EMI/RFI lots of moving parts are involved. Generally speaking metal abrasion is the culprit in these scenarios.

In addition to the metallic parts causing that "twitch" you could also get it within specific throttle ranges. In a manner similar to shimmy on a set of tires on an automobile within certain speed ranges, the combination of rotating parts (as driven at a given throttle range) can create the aforementioned difficulties. Does the problem appear only at a specific throttle setting?

It does not appear to be related to any particular throttle setting. I won't be able to try anything suggested so far until the weekend. Thanks for all of the advice so far.