Hi.

I don't have and have never used any version of ACID so what I write below is based upon assumptions made from what you write.

Since your problem is manifested on MIDI rather than Audio tracks, no amount of changes to buffer settings or drivers is going to fix your problem, I think.

From what you write I assume you have several MIDI tracks being "played back" at the same time as you are recording a new one. Is that correct? How many tracks? Are you using internal (to the computer) VSTis as the "sound generator" or external modules or keyboards?

I ask all that because, to me, it sounds as if your MIDI data stream is being asked to handle too many simultaneous events. As you know, MIDI can handle up to 16 separate data channels in an ideal environment; in practice, you may not actually achieve that in the real world.

I'm not sure how to resolve this problem if you are using entirely internal VSTis; perhaps your computer is not really fast enough to process the data in real time? If you are using external modules or keyboards, the easiest way to solve this problem is to use a multi-output MIDI "interface" that will allow several "sets" of 16 MIDI channels to operate simultaneously. In my case I use an external Midisport 4 x 4 USB interface; this gives me the option of 4 separate MIDI inputs (in use I rarely need more than one) but also 4 separate MIDI outputs, thus allowing me to have different keyboards/modules on different MIDI connections so removing the necessity to ever use MIDI THRU (which can itself prove a bottleneck under "heavy use"). The connection to the computer is via USB2, using specialist drivers.

HTH

Jeff