you could have a leak at in the inlet manifold or in one of the vacuum hoses. its easiest to just remove it renew the seals and use a good sealant when you put it all back on.

another thing to try would be to use a short section of silicon hose between the filter and the MAF to increase the distance between the two. My 2 litre had a similar problem when i fitted a BMC. Turned out that the filter was creating too much turbulence and 'confusing' the MAF. After I increased the distance between the two points problem gone.

As a point of precaution just check you tightened everything back together to make sure you havnt got any leaks in the system.

Hope this Helps