Another shot in the dark but have you had a look at air & fuel filters, you can tell by looking at the air filter, if it's filthy then change it,
Fuel filter is a bit more tricky and dirty to examine.
If engine is failing to fire at any point it must be lack of air, fuel or spark. My last car was fine when driving gently but misfiring under load, turned out to be sludge in the fuel filter.
Good luck.