bash up the performance monitor and watch it carefully when you record - if the red light comes on, then you have problems. You could be taxing the processor, or taxing the drive's in/out performance. I get this from time to time, when I have busy tracks and processing power runs out, the buffers empty and you get a nasty glitch. If it has always been good and suddenly is playing up, then it's likely there is another programme running that is making demands on the processor and/or drives. I'd left windows update on, and it suddenly burst into life halfway through a recording and wrecked it. AVG that I use often seems to suddenly grab the internet and update stuff - essential thing, I guess, but annoying. If you have software synths or samplers and run multiple instances these can suddenly all grab their files from the drive at the same time and max out the system.
If your computer is playing one track and recording another, and makes the nasty spike - then something else is causing it - external to Cubase.ctrl-alt-del and look at task manager to see what is running may point you in the right direction?