my 1977 xj6 4.2 has a loss of power if i go over 60mph and starts to misfire, it also does not shut off right away when i switch the ignition off. any ideas on where i should start to look first?

 

First guess is the fuel pump/s are getting tired and cannot supply enough fuel for that engine rev/load, or the fuel filter is blocked, same result.

1977 should have twin pumps, one inside each tank. Swap tanks, IF that is your cars spec, and see if the fault is on both tanks, if so, the fuel filter is my best guess, coz 2 pumps do not normally die at the same time. If the car is a single pump spec, back to my first statement.

Of course there may be carby issues, spark plugs worn out, etc etc, but simple things first sometimes pays dividends.

 

Sounds like the ignition timing is off

 

I'd check the timing first and then the air filter. Both cost nothing to check and both could easily be the cause. The engine running on will be hot exhaust valves again suggesting timing issues