Wilko makes a good point (pun intended) about timing having changed.
Check points gap; should be between 0.012" - 0.015".
If the adjustment nut(s) work loose, the gap and timing will change, and the performance will change quite suddenly, and suffer.
I have had this happen. I now always include a set spanner in the tool kit which matches the nuts (from memory; 1/4" AF) because it is difficult to re-adjust points by the side of the road, without the right tool!
Also, the bakelite heel which follows the cam profile can also wear, with the same result.
Richard