1992 XJ6 Sovereign 4.0L Was Driving fine, next time using slow reaching drive speed (no power) when it reaches 50 mph engine drops to idle with no throttle speed goes down to 15 mph. If you shut off engine & restart you can slowly get back up to 50 mph and it does the ssame thing. No codes given. If in park or neutral the throttle works normal, either with foot peddle or by hand under hood ? ? ?

 

I read a recent thread about tranny fluid level affecting engine running - check the level - warm up car, move gear selector around through all gears then dip fluid.

Just a WAG but worth a try