a little help please

for the second time now, and the two times have been sparated by about 2 months, when going in reverse my XJ cuts out. It the engine just stops. the second time I was able to shift into drive and not have any problems, but the first time the RPMs would just shoot up an down while in drive. Im thinking something along the lines of fuel injection but I do not know? any idea??

you might check to see if the bolts are tight on the trans to the engine.
If the crankshaft sensor is moving around, it will quit. Just a wild guess.
Big O

Check the TPS, may be going bad