I'm with Bob here.
If pressing "E" clears it, then look for any effects that are cpu intensive, or use look-ahead, and turn them off until you finish tracking.
In Sonar typical culprits are: Perfect Space, LP-64 Multiband, LP-64 EQ, TS-64, PC Concrete Limiter, Boost 11.
There are others, but these are the ones I seem to see mentioned on here.