'97 with electrical problems

I have a '97 Jeep Wrangler and I am having problems that noone is able to figure out. Every once in a while when driving and I hit a bump the gas gauge goes to empty and the rpms go to zero. Sometimes the jeep stalls. I brought it to a Jeep dealer to have the power steering pump replaced and told them about the problem and they said that they tried it out and had it bumping all around and nothing happened so they dismissed it. Sometimes when the jeep dies and you have to restart it. This is a very frustrating problem because I don't want to be driving on the highway and have the thing shut off on me. The mechanic that is looking at it now has said that all of the wiring that they have looked at looks fine, but they need to look longer to see if they can find the problem. I was wondering if it could be something else. I have had the catalitic converter replaced because that got all clogged up and also had the gas tank cleaned out and there was ice and a lot of gunk in there but I am stumped.
Any suggestions so I can tell my mechanic what to look at?
Thanks

I'd think it might be something up under the dash that is loose enough to be dislodged when you hit a bump -- based on gas guage and tach. I'd start there. Have your mechanic test the entire guage cluster too.