Unless the compression has been increased, the engine should be OK on mid grade fuel. I run my TR4 on 97octane but it is a higher compression engine. Could be that your timing is too far advanced, but if you want some reading to reassure you that running on is a common problem, try this link: (Post No 13 is helpful.)
http://www.tr-register.co.uk/forums/index.php?showtopic=35035Having said all that you should try to eliminate it, and a higher octane fuel may help whilst you get it set up.
Malcolm
