Just tried Cobra original release (Cobra.tap.zip from WOS) and the game play speed does repeatedly speed up and slow down. I wonder if this could be something to do with floating bus behavior. I believe Joffa reads the floating bus in a few of his games (Instead of counting the cycles after the vertical retrace interrupt) to work out where the raster is. I'm just guessing now (or I might have read it somewhere), but maybe he did away with waiting for the vertical refresh interrupt completely and relied solely on the floating bus for Cobra's in game timing. Unfortunately, mame doesn't emulate floating bus behavior accurately yet though.
However, there are a few versions of Cobra on WOS. Some labeled as 'fixed', and a few re-releases (The Hit Squad). Another guess, but I think the newer versions don't rely on floating bus behavior (So that they can work on newer Spectrum models +2a and +3, where there is no floating bus). I just tried the one called Cobra(TheHitSquad).tzx.zip and this one looks & plays fine on mame